Atomic theory is the scientific theory that matter is composed of particles called atoms. Atomic theory traces its origins to an ancient philosophical tradition known as atomism. According to this idea, if one were to take a lump of matter and cut it into ever smaller pieces, one would eventually reach a point where the pieces could not be further cut into anything smaller. Ancient Greek philosophers called these hypothetical ultimate particles of matter "atomos", a word which meant "uncut". In the early 1800s, the scientist John Dalton noticed that chemical substances seemed to combine and break down into other substances by weight in proportions that suggested that each chemical element is ultimately made up of tiny indivisible particles of consistent weight.
Introduction
Atomic theory
Atomic theory | Introduction
John Dalton Near the end of the 18th century The Pre-Socratic Greek philosophers Leucippus and his pupil Democritus. John Dalton.
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Atomic theory is the scientific theory that matter is composed of particles called atoms. Atomic theory traces its origins to an ancient philosophical tradition known as atomism. According to this idea, if one were to take a lump of matter and cut it into ever smaller pieces, one would eventually reach a point where the pieces could not be further cut into anything smaller. Ancient Greek philosophers called these hypothetical ultimate particles of matter "atomos", a word which meant "uncut". In the early 1800s, the scientist John Dalton noticed that chemical substances seemed to combine and break down into other substances by weight in proportions that suggested that each chemical element is ultimately made up of tiny indivisible particles of consistent weight.
Atomic theory | Introduction
The theory of Democritus held that everything is composed of "atoms," which are physically, but not geometrically, indivisible; that between atoms, there lies empty space; that atoms are indestructible, and have always been and always will be in motion; that there is an infinite number of atoms and of kinds of atoms, which differ in shape and size. Of the mass of atoms, Democritus said, "The more any indivisible exceeds, the heavier it is." However, his exact position on atomic weight is disputed. Leucippus is widely credited with having been the first to develop the theory of atomism, although Isaac Newton preferred to credit the obscure Mochus the Phoenician (whom he believed to be the biblical Moses) as the inventor of the idea on the authority of Posidonius and Strabo.
Democritus | Atomic hypothesis
A study of Dalton's laboratory notebooks, discovered in the rooms of the Manchester Literary and Philosophical Society, concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced on him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper "On the Absorption of Gases by Water and other Liquids" already mentioned.
John Dalton | Atomic theory
By the early 20th century, scientists had developed fairly detailed and precise models for the structure of matter, which led to more rigorously-defined classifications for the tiny invisible particles that make up ordinary matter. An "atom" is now defined as the basic particle that composes a chemical element. Around the turn of the 20th century, physicists discovered that the particles that chemists called "atoms" are in fact agglomerations of even smaller particles ("subatomic" particles), but scientists kept the name out of convention. The term "elementary particle" is now used to refer to particles that are actually indivisible.
Atomic theory | Introduction
The "Stanford Encyclopedia of Philosophy" notes, "This theologically motivated view does not seem to claim much historical evidence, however". Democritus, along with Leucippus and Epicurus, proposed the earliest views on the shapes and connectivity of atoms. They reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials. Using analogies from humans' sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts.
Democritus | Atomic hypothesis
In 1913, Niels Bohr incorporated this idea into his Bohr model of the atom, in which an electron could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus (i.e., their radii) being proportional to its energy. Under this model an electron could not spiral into the nucleus because it could not lose energy in a continuous manner; instead, it could only make instantaneous "quantum leaps" between the fixed energy levels. When this occurred, light was emitted or absorbed at a frequency proportional to the change in energy (hence the absorption and emission of light in discrete spectra).
Atomic theory | First steps toward a quantum physical model of the atom
Near the end of the 18th century, two laws about chemical reactions emerged without referring to the notion of an atomic theory. The first was the law of conservation of mass, closely associated with the work of Antoine Lavoisier, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). The second was the law of definite proportions. First established by the French chemist Joseph Proust in 1797 this law states that if a compound is broken down into its constituent chemical elements, then the masses of the constituents will always have the same proportions by weight, regardless of the quantity or source of the original substance.
Atomic theory | John Dalton
One of the earliest steps towards atomic physics was the recognition that matter was composed of "atoms". It forms a part of the texts written in 6th century BC to 2nd century BC such as those of Democritus or Vaisheshika Sutra written by Kanad. This theory was later developed in the modern sense of the basic unit of a chemical element by the British chemist and physicist John Dalton in the 18th century. At this stage, it wasn't clear what atoms were although they could be described and classified by their properties (in bulk). The invention of the periodic system of elements by Mendeleev was another great step forward.
Atomic physics | History and developments
The idea that matter is made up of discrete units is a very old idea, appearing in many ancient cultures such as Greece and India. The word "atom" (; ""), meaning "uncuttable", was coined by the Pre-Socratic Greek philosophers Leucippus and his pupil Democritus (460–370 BC). Democritus taught that atoms were infinite in number, uncreated, and eternal, and that the qualities of an object result from the kind of atoms that compose it. Democritus's atomism was refined and elaborated by the later Greek philosopher Epicurus (341–270 BC), and by the Roman Epicurean poet Lucretius (99–55 BC). During the Early Middle Ages, atomism was mostly forgotten in western Europe.During the 12th century, atomism became known again in western Europe through references to it in the newly-rediscovered writings of Aristotle.
Atomic theory | Philosophical atomism
Shortly after 1850, certain physicists developed the kinetic theory of gases and of heat, which mathematically modelled the behavior of gases by assuming that they were made of particles. In the early 20th century, Albert Einstein and Jean Perrin proved that Brownian motion (the erratic motion of pollen grains in water) is caused by the action of water molecules; this third line of evidence silenced remaining doubts among scientists as to whether atoms and molecules were real. Throughout the nineteenth century, some scientists had cautioned that the evidence for atoms was indirect, and therefore atoms might not actually be real, but only seem to be real.
Atomic theory | Introduction
The concept of 'atom' proposed by Democritus was an early philosophical attempt to unify phenomena observed in nature. The concept of 'atom' also appeared in the Nyaya-Vaisheshika school of ancient Indian philosophy. Archimedes was possibly the first philosopher to have described nature with axioms (or principles) and then deduce new results from them. Any "theory of everything" is similarly expected to be based on axioms and to deduce all observable phenomena from them.
Theory of everything | Antiquity to 19th century
By the late 18th century, the useful practices of engineering and technology began to influence philosophical explanations for the composition of matter. Those who speculated on the ultimate nature of matter began to verify their "thought experiments" with some repeatable demonstrations, when they could. Roger Boscovich provided the first general mathematical theory of atomism, based on the ideas of Newton and Leibniz but transforming them so as to provide a programme for atomic physics. In 1808, John Dalton assimilated the known experimental work of many people to summarize the empirical evidence on the composition of matter. He noticed that distilled water everywhere analyzed to the same elements, hydrogen and oxygen.
Atomism | Modern atomic theory
One of the earliest steps towards "atomic physics" was the recognition that matter was composed of "atoms", in modern terms the basic unit of a chemical element. This theory was developed by John Dalton in the 18th century. At this stage, it wasn't clear what atoms were - although they could be described and classified by their observable properties in bulk; summarized by the developing periodic table, by John Newlands and Dmitri Mendeleyev around the mid to late 19th century. Later, the connection between atomic physics "and" optical physics became apparent, by the discovery of spectral lines and attempts to describe the phenomenon - notably by Joseph von Fraunhofer, Fresnel, and others in the 19th century.
Atomic, molecular, and optical physics | History
He hypothesized that there existed sub-atomic particles of unit charge; one of the first persons ever to do so. He suggested that the atom was made up of a core of material surrounded by concentric shells of these electrical 'atoms', or particles. He also believed that these particles could be added or subtracted to an atom, changing its charge.
Richard Laming | Introduction
Similar to these views, in 1803 John Dalton took the atomic weight of hydrogen, the lightest element, as unity, and determined, for example, that the ratio for nitrous anhydride was 2 to 3 which gives the formula NO. Dalton incorrectly imagined that atoms "hooked" together to form molecules. Later, in 1808, Dalton published his famous diagram of combined "atoms": Amedeo Avogadro created the word "molecule". His 1811 paper "Essay on Determining the Relative Masses of the Elementary Molecules of Bodies", he essentially states, i.e. according to Partington's "A Short History of Chemistry", that: Note that this quote is not a literal translation.
History of molecular theory | 19th century
The earliest views on the shapes and connectivity of atoms was that proposed by Leucippus, Democritus, and Epicurus who reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials. It was Democritus that was the main proponent of this view. Using analogies based on the experiences of the senses, he gave a picture or an image of an atom in which atoms were distinguished from each other by their shape, their size, and the arrangement of their parts.
History of molecular theory | 17th century
The nature of this differing mass would later be explained by the discovery of neutrons in 1932.
Atomic theory | Discovery of isotopes
In his proposal there would have been a point, which had been called a Universon, that would have collapsed into a Cosmon and an Anticosmon pair. Goldhaber was wondering about why is there any matter if equal amount of matter and antimatter was formed in the beginning of the big bang. One explanation for this is the asymmetry of matter meaning that there could have been slightly more matter than antimatter, for instance 1001 matter particles to every 1000 antimatter. In Goldhabers model cosmon and anticosmon would have flown apart and therefore explaining issue without asymmetry. In 1989 Hans Dehmelt attempted to modernize the idea of the primeval atom.
Cosmon | Introduction
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John Dalton | Atomic theory
It is not necessary for the core to circulate, as it did in the Cartesian model. Helmholtz also showed that vortices exert forces on one another, and those forces take a form analogous to the magnetic forces between electrical wires. During the intervening period, chemist John Dalton had developed his atomic theory of matter. It remained only to bring the two strands of discovery together.
Vortex theory of the atom | Origins
He noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump. At around 450 BCE Democritus, a later Greek philosopher, developed an atomic theory that was similar to modern atomic theory. His mentor, Leucippus, is credited with this same theory. The hypothesis of Leucippus and Democritus held everything to be composed of atoms. But these "atoms", called "atomos", were indivisible, and indestructible. He presciently stated that between atoms lies empty space, and that atoms are constantly in motion. He was incorrect only in stating that atoms come in different sizes and shapes, and that each object had its own shaped and sized atom.
History of electrical engineering | Ancient developments
In 1803, English meteorologist and chemist John Dalton proposed Dalton's law, which describes the relationship between the components in a mixture of gases and the relative pressure each contributes to that of the overall mixture. Discovered in 1801, this concept is also known as Dalton's law of partial pressures. Dalton also proposed a modern atomic theory in 1803 which stated that all matter was composed of small indivisible particles termed atoms, atoms of a given element possess unique characteristics and weight, and three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). In 1808, Dalton first published "New System of Chemical Philosophy" (1808-1827), in which he outlined the first modern scientific description of the atomic theory.
History of chemistry | John Dalton
Irving Langmuir was the first to propose in his 1919 article "The Arrangement of Electrons in Atoms and Molecules" in which, building on Gilbert N. Lewis's cubical atom theory and Walther Kossel's chemical bonding theory, he outlined his "concentric theory of atomic structure". Langmuir had developed his work on electron atomic structure from other chemists as is shown in the development of the History of the periodic table and the Octet rule. Niels Bohr (1923) incorporated Langmuir’s model that the periodicity in the properties of the elements might be explained by the electronic structure of the atom. His proposals were based on the then current Bohr model of the atom, in which the electron shells were orbits at a fixed distance from the nucleus.
Electron configuration | History
He measured the mass-to-charge ratio and discovered it was 1800 times smaller than that of hydrogen, the smallest atom. These corpuscles were a particle unlike any other previously known. Thomson suggested that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model as the electrons were embedded in the positive charge like raisins in a plum pudding (although in Thomson's model they were not stationary).
Atomic theory | Discovery of subatomic particles
Antonius Johannes van den Broek (4 May 1870, Zoetermeer – 25 October 1926, Bilthoven) was a Dutch amateur physicist notable for being the first who realized that the number of an element in the periodic table (now called atomic number) corresponds to the charge of its atomic nucleus. This hypothesis was published in 1911 and inspired the experimental work of Henry Moseley, who found good experimental evidence for it by 1913.
Antonius van den Broek | Introduction
Leucippus (; , "Leúkippos"; fl. 5th cent. BCE) is reported in some ancient sources to have been a philosopher who was the earliest Greek to develop the theory of atomism—the idea that everything is composed entirely of various imperishable, indivisible elements called atoms. Leucippus often appears as the master to his pupil Democritus, a philosopher also touted as the originator of the atomic theory. A brief notice in Diogenes Laërtius’s life of Epicurus says that on the testimony of Epicurus, Leucippus never existed. As the philosophical heir of Democritus, Epicurus's word has some weight, and indeed a controversy over this matter raged in German scholarship for many years at the close of the 19th century.
Leucippus | Introduction
Between 1908 and 1913, Rutheford and his colleagues performed a series of experiments in which they bombarded thin foils of metal with alpha particles. They spotted alpha particles being deflected by angles greater than 90°. To explain this, Rutherford proposed that the positive charge of the atom is not distributed throughout the atom's volume as Thomson believed, but is concentrated in a tiny nucleus at the center. Only such an intense concentration of charge could produce an electric field strong enough to deflect the alpha particles as observed.
Atomic theory | Discovery of the nucleus
He realized that the plum pudding model could not be accurate and that the deflections of the alpha particles could only be explained if the positive and negative charges were separated from each other and that the mass of the atom was a concentrated point of positive charge. This justified the idea of a nuclear atom with a dense center of positive charge and mass.
Atomic nucleus | History
The chemist Robert Boyle (1627–1691) and the physicist Isaac Newton (1642–1727) both defended atomism and, by the end of the 17th century, it had become accepted by portions of the scientific community.
Atomic theory | Philosophical atomism
However, recent evidence suggests that Dalton's development of thought may have been influenced by the ideas of another Irish chemist Bryan Higgins, who was William's uncle. Bryan believed that an atom was a heavy central particle surrounded by an atmosphere of caloric, the supposed substance of heat at the time. The size of the atom was determined by the diameter of the caloric atmosphere. Based on the evidence, Dalton was aware of Bryan's theory and adopted very similar ideas and language, but he never acknowledged Bryan's anticipation of his caloric model. However, the essential novelty of Dalton's atomic theory is that he provided a method of calculating relative atomic weights for the chemical elements, something that neither Bryan nor William Higgins did; his priority for that crucial step is uncontested.
John Dalton | Atomic theory
The particles of chemical matter for which chemists and other natural philosophers of the early 19th century found experimental evidence were thought to be indivisible, and therefore were given by John Dalton the name "atom", long used by the atomist philosophy. Although the connection to historical atomism is at best tenuous, elementary particles have become a modern analogue of philosophical atoms.
Atomism | Introduction
These results refined Ernest Rutherford's and Antonius van den Broek's model, which proposed that the atom contains in its nucleus a number of positive nuclear charges that is equal to its (atomic) number in the periodic table. Until these experiments, atomic number was not known to be a physical and experimental quantity. That it is equal to the atomic nuclear charge remains the accepted atomic model today. Chemical bonds between atoms were explained by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons. As the chemical properties of the elements were known to largely repeat themselves according to the periodic law, in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner.
Atom | Bohr model
In 1905, Albert Einstein proved the reality of these molecules and their motions by producing the first statistical physics analysis of Brownian motion. French physicist Jean Perrin used Einstein's results to experimentally determine the mass, and the dimensions, of atoms, thereby conclusively verifying Dalton's atomic theory. Before this paper, atoms were recognized as a useful concept, but physicists and chemists hotly debated whether atoms were real entities. Einstein's statistical discussion of atomic behavior gave experimentalists a way to count atoms with an ordinary microscope. Wilhelm Ostwald, one of the leaders of the anti-atom school, later told Arnold Sommerfeld that he had been converted to a belief in atoms by Einstein's complete explanation of Brownian motion.
Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen | Introduction
Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets (see diagram). With the rise of scholasticism and the decline of the Roman Empire, the atomic theory was abandoned for many ages in favor of the various four element theories and later alchemical theories. The 17th century, however, saw a resurgence in the atomic theory primarily through the works of Gassendi, and Newton. Among other scientists of that time Gassendi deeply studied ancient history, wrote major works about Epicurus natural philosophy and was a persuasive propagandist of it.
History of molecular theory | 17th century
Prout's hypothesis was an early 19th-century attempt to explain the existence of the various chemical elements through a hypothesis regarding the internal structure of the atom. In 1815 and 1816, the English chemist William Prout published two papers in which he observed that the atomic weights that had been measured for the elements known at that time appeared to be whole multiples of the atomic weight of hydrogen. He then hypothesized that the hydrogen atom was the only truly fundamental object, which he called protyle, and that the atoms of other elements were actually groupings of various numbers of hydrogen atoms.
Prout's hypothesis | Introduction
The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, independently appeared in ancient Greece and ancient India among Buddhists, Hindus and Jains in 1st-millennium BC. Ancient philosophers who proposed the particulate theory of matter include Kanada (c. 6th–century BC or after), Leucippus (~490 BC) and Democritus (~470–380 BC).
Matter | Introduction
In the early 20th century, the American chemist Gilbert N. Lewis began to use dots in lecture, while teaching undergraduates at Harvard, to represent the electrons around atoms. His students favored these drawings, which stimulated him in this direction. From these lectures, Lewis noted that elements with a certain number of electrons seemed to have a special stability. This phenomenon was pointed out by the German chemist Richard Abegg in 1904, to which Lewis referred to as "Abegg's law of valence" (now generally known as Abegg's rule). To Lewis it appeared that once a core of eight electrons has formed around a nucleus, the layer is filled, and a new layer is started.
History of molecular theory | 20th century
Chemical bonds between atoms were explained by Gilbert Newton Lewis, who in 1916 proposed that a covalent bond between two atoms is maintained by a pair of electrons shared between them. Later, in 1927, Walter Heitler and Fritz London gave the full explanation of the electron-pair formation and chemical bonding in terms of quantum mechanics. In 1919, the American chemist Irving Langmuir elaborated on the Lewis' static model of the atom and suggested that all electrons were distributed in successive "concentric (nearly) spherical shells, all of equal thickness". In turn, he divided the shells into a number of cells each of which contained one pair of electrons.
Electron | Atomic theory
To admit this theory would be to deny the immortality of the soul (see Alexander of Aphrodisias). Wherefore the Motekallamin had, before anything else, to establish a system of philosophy to demonstrate the creation of matter, and they adopted to that end the theory of atoms as enunciated by Democritus. They taught that atoms possess neither quantity nor extension. Originally atoms were created by God, and are created now as occasion seems to require. Bodies come into existence or die, through the aggregation or the sunderance of these atoms. But this theory did not remove the objections of philosophy to a creation of matter.
Judeo-Islamic philosophies (800–1400) | Argument for creation
But if, on the contrary, you want to spend at least your first few months patiently, comfortably, and joyfully in discussions that keep coming back to the same few points, chatting about a few basic questions with me and our young people—and without thinking much about publishing (!!!)—why then I welcome you with open arms!! Characteristically, he did not like the abstraction of the new quantum theory of Heisenberg and Dirac. Ehrenfest's most important contribution from 1912 to 1933 is the theory of adiabatic invariants. It is a concept derived from classical mechanics that on the one hand can serve to refine certain methods of Bohr's model of the atom (although initially Ehrenfest did not accept Bohr's ideas) and on the other hand makes a link between atomic mechanics and statistical mechanics.
Paul Ehrenfest | Research
Around 420 BC, Empedocles stated that all matter is made up of four elemental substances: earth, fire, air and water. The early theory of atomism can be traced back to ancient Greece and ancient India. Greek atomism dates back to the Greek philosopher Democritus, who declared that matter is composed of indivisible and indestructible particles called "atomos" around 380 BC. Leucippus also declared that atoms were the most indivisible part of matter. This coincided with a similar declaration by Indian philosopher Kanada in his Vaisheshika sutras around the same time period. In much the same fashion he discussed the existence of gases.
History of chemistry | Ancient world
John Dalton studied and expanded upon this previous work and defended a new idea, later known as the law of multiple proportions: if the same two elements can be combined to form a number of different compounds, then the ratios of the masses of the two elements in their various compounds will be represented by small whole numbers. This is a common pattern in chemical reactions that was observed by Dalton and other chemists at the time. Example 1 — tin oxides: Dalton identified two oxides of tin. One is a grey powder in which for every 100 parts of tin there is 13.5 parts of oxygen.
Atomic theory | John Dalton
Despite these discoveries, the term "atom" stuck to Dalton's (chemical) atoms and now denotes the smallest particle of a chemical element, not something really indivisible.
History of subatomic physics | First subatomic particles
The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on a simplistic interpretation of early values of atomic weights (see Prout's hypothesis), which was disproved when more accurate values were measured. In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J.
Proton | History
In 1827, the British botanist Robert Brown observed that dust particles inside pollen grains floating in water constantly jiggled about for no apparent reason. In 1905, Albert Einstein theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it. This model was validated experimentally in 1908 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory).
Atomic theory | Brownian Motion
Gold began discussing problems in physics with Hoyle and Bondi again, centering on the issues over redshift and Hubble's law. This led the three to all start questioning the Big Bang theory originally proposed by Georges Lemaître in 1931 and later advanced by George Gamow, which suggested that the universe expanded from an extremely dense and hot state and continues to expand today. As recounted in a 1978 interview with physicist and historian Spencer R. Weart, Gold believed that there was reason to think that the creation of matter was "done all the time and then none of the problems about fleeting moments arise.
Thomas Gold | Steady-state theory
In the 14th century, the rediscovery of major works describing atomist teachings, including Lucretius's "De rerum natura" and Diogenes Laërtius's "Lives and Opinions of Eminent Philosophers", led to increased scholarly attention on the subject. Nonetheless, because atomism was associated with the philosophy of Epicureanism, which contradicted orthodox Christian teachings, belief in atoms was not considered acceptable by most European philosophers. The French Catholic priest Pierre Gassendi (1592–1655) revived Epicurean atomism with modifications, arguing that atoms were created by God and, though extremely numerous, are not infinite and the first person who use term "molecule" to describe aggregate of atom. Gassendi's modified theory of atoms was popularized in France by the physician François Bernier (1620–1688) and in England by the natural philosopher Walter Charleton (1619–1707).
Atomic theory | Philosophical atomism
That proposal was formally adopted by the International Committee on Atomic Weights (ICAW) in 1903. That was approximately the mass of one hydrogen atom, but oxygen was more amenable to experimental determination. This suggestion was made before the discovery of the existence of elemental isotopes, which occurred in 1912. The same definition was adopted in 1909 the physicist Jean Perrin in his extensive experiments to determine the atomic masses and Avogadro's constant. This definition remained unchanged until 1961. Perrin also defined the "mole" as an amount of a compound that contained as many molecules as 32 grams of oxygen ().
Dalton (unit) | Origin of the concept
About 1902 Lewis started to use unpublished drawings of cubical atoms in his lecture notes, in which the corners of the cube represented possible electron positions. Lewis later cited these notes in his classic 1916 paper on chemical bonding, as being the first expression of his ideas. A third major interest that originated during Lewis’ Harvard years was his valence theory. In 1902, while trying to explain the laws of valence to his students, Lewis conceived the idea that atoms were built up of a concentric series of cubes with electrons at each corner. This “cubic atom” explained the cycle of eight elements in the periodic table and was in accord with the widely accepted belief that chemical bonds were formed by transfer of electrons to give each atom a complete set of eight.
Gilbert N. Lewis | Valence theory
One of Democritus' arguments in support of atomism was that atoms naturally explain the sharp phase boundaries observed in materials, as when ice melts to water or water turns to steam. His idea was that small changes in atomic-scale properties would lead to big changes in the aggregate behavior. Others believed that matter is inherently continuous, not atomic, and that the large-scale properties of matter are not reducible to basic atomic properties. While the laws of chemical binding made it clear to nineteenth century chemists that atoms were real, among physicists the debate continued well into the early twentieth century.
Ising model | Historical significance
He reasoned that to account for the size and shape of atoms moving in a void could account for the properties of matter. Heat was due to small, round atoms; cold, to pyramidal atoms with sharp points, which accounted for the pricking sensation of severe cold; and solids were held together by interlacing hooks. Newton, though he acknowledged the various atom attachment theories in vogue at the time, i.e. "hooked atoms", "glued atoms" (bodies at rest), and the "stick together by conspiring motions" theory, rather believed, as famously stated in "Query 31" of his 1704 "Opticks", that particles attract one another by some force, which "in immediate contact is extremely strong, at small distances performs the chemical operations, and reaches not far from particles with any sensible effect." In a more concrete manner, however, the concept of aggregates or units of bonded atoms, i.e.
History of molecular theory | 17th century
This is the idea put forward by Van den Broek. The number of the place of an element in the periodic table (or atomic number) at that time was not thought by most physicists to be a physical property. It was not until the work of Henry Moseley working with the Bohr model of the atom with the explicit idea of testing Van den Broek's theory, that it was realized that atomic number was indeed a purely physical property (the charge of the nucleus) which could be measured, and that Van den Broek's original guess had been correct, or very close to being correct.
Antonius van den Broek | Major contribution to science
In the 5th century BCE, Leucippus and his pupil Democritus proposed that all matter was composed of small indivisible particles called atoms. Nothing whatsoever is known about Leucippus except that he was the teacher of Democritus. Democritus, by contrast, was a prolific writer, who wrote over eighty known treatises, none of which have survived to the present day complete. However, a massive number of fragments and quotations of his writings have survived. These are the main source of information on his teachings about atoms. Democritus's argument for the existence of atoms hinged on the idea that it is impossible to keep dividing matter for infinity and that matter must therefore be made up of extremely tiny particles.
Atomism | Greek atomism
There is then no need to interrupt the sequence or move the f block into a 'footnote'. He believed that no elements heavier than number 120 would be found, so he did not envisage a g block. In terms of atomic quantum numbers, each row corresponds to one value of the sum where n is the principal quantum number and ℓ the azimuthal quantum number. The table therefore corresponds to the Madelung rule, which states that atomic subshells are filled in order of increasing values of . Janet also envisaged an element zero whose 'atom' would consist of two neutrons, and he speculated that this would be the link to a mirror-image table of elements with negative atomic numbers – in effect anti-matter.
Charles Janet | Chemical ideas
Important physical and mathematical traditions also existed in ancient Chinese and Indian sciences. In Indian philosophy, Maharishi Kanada was the first to systematically develop a theory of atomism around 200 BCE though some authors have allotted him an earlier era in the 6th century BCE. It was further elaborated by the Buddhist atomists Dharmakirti and Dignāga during the 1st millennium CE. Pakudha Kaccayana, a 6th-century BCE Indian philosopher and contemporary of Gautama Buddha, had also propounded ideas about the atomic constitution of the material world. These philosophers believed that other elements (except ether) were physically palpable and hence comprised minuscule particles of matter.
History of physics | India and China
As mentioned above, the term "logical atomism" was first coined by Russell in 1911. However, since the paper in which it was first introduced was published only in French during Russell's lifetime, the view was not widely associated with Russell in the English-speaking world until Russell gave a series of lectures in early 1918 under the title "The Philosophy of Logical Atomism". These lectures were subsequently published in 1918 and 1919 in "The Monist" (Volumes 28 and 29), which at the time was edited by Phillip Jourdain. Russell's ideas as presented in 1918 were also influenced by Wittgenstein, as he explicitly acknowledges in his introductory note.
Logical atomism | Origin
In 1915, the reason for nuclear charge being quantized in units of "Z", which were now recognized to be the same as the element number, was not understood. An old idea called Prout's hypothesis had postulated that the elements were all made of residues (or "protyles") of the lightest element hydrogen, which in the Bohr-Rutherford model had a single electron and a nuclear charge of one. However, as early as 1907, Rutherford and Thomas Royds had shown that alpha particles, which had a charge of +2, were the nuclei of helium atoms, which had a mass four times that of hydrogen, not two times.
Atomic number | The proton and the idea of nuclear electrons
Atomism is a central part of today's relationship between thermodynamics and statistical mechanics. Ancient thinkers such as Leucippus and Democritus, and later the Epicureans, by advancing atomism, laid the foundations for the later atomic theory. Until experimental proof of atoms was later provided in the 20th century, the atomic theory was driven largely by philosophical considerations and scientific intuition.
History of thermodynamics | Contributions from antiquity
The interpretation of the law of definite proportions in terms of the atomic theory of matter implied that the masses of atoms of various elements had definite ratios that depended on the elements. While the actual masses were unknown, the relative masses could be deduced from that law. In 1803 John Dalton proposed to use the (still unknown) atomic mass of the lightest atom, that of hydrogen, as the natural unit of atomic mass. This was the basis of the atomic weight scale. For technical reasons, in 1898, chemist Wilhelm Ostwald and others proposed to redefine the unit of atomic mass as 1/16 of the mass of an oxygen atom.
Dalton (unit) | Origin of the concept
The history of the mole is intertwined with that of molecular mass, atomic mass units, and the Avogadro number. The first table of standard atomic weight (atomic mass) was published by John Dalton (1766–1844) in 1805, based on a system in which the relative atomic mass of hydrogen was defined as 1. These relative atomic masses were based on the stoichiometric proportions of chemical reaction and compounds, a fact that greatly aided their acceptance: It was not necessary for a chemist to subscribe to atomic theory (an unproven hypothesis at the time) to make practical use of the tables. This would lead to some confusion between atomic masses (promoted by proponents of atomic theory) and equivalent weights (promoted by its opponents and which sometimes differed from relative atomic masses by an integer factor), which would last throughout much of the nineteenth century.
Mole (unit) | History
In response to Parmenides definition of motion, Democritus (born circa 460) expounded the atomic theory, in which indivisible bits of matter are in constant motion through the void. In the absence of something to perturb them they fall evenly through space. According to this school of thought matter and or space (and time) are discrete and finite. Evidence for this theory was found by John Dalton in the early 1800s, explaining the finding that chemical decomposition of compounds gives whole numbered ratios of weight, leading to Dalton's atomic theory Motion conceived in this way led to the approach of determinism and therefore to questioning how free will could exist.
Philosophy of motion | Atomism and determinism
Thomson had encountered a similar problem in his work on cathode rays, which he solved by creating a near-perfect vacuum in his instruments. Rutherford didn't think he'd run into this same problem because alpha particles are much heavier than electrons. According to Thomson's model of the atom, the positive charge in the atom is not concentrated enough to produce an electric field strong enough to deflect an alpha particle, and the electrons are so lightweight they should be pushed aside effortlessly by the much heavier alpha particles. Yet there was scattering, so Rutherford and his colleagues decided to investigate this scattering carefully.
Atomic theory | Discovery of the nucleus
In 1925, they suggested that an electron, in addition to the angular momentum of its orbit, possesses an intrinsic angular momentum and magnetic dipole moment. This is analogous to the rotation of the Earth on its axis as it orbits the Sun. The intrinsic angular momentum became known as spin, and explained the previously mysterious splitting of spectral lines observed with a high-resolution spectrograph; this phenomenon is known as fine structure splitting.
Electron | Atomic theory
Bohr's model was not perfect. It could only predict the spectral lines of hydrogen; it couldn't predict those of multielectron atoms. Worse still, as spectrographic technology improved, additional spectral lines in hydrogen were observed which Bohr's model couldn't explain. In 1916, Arnold Sommerfeld added elliptical orbits to the Bohr model to explain the extra emission lines, but this made the model very difficult to use, and it still couldn't explain more complex atoms.
Atomic theory | First steps toward a quantum physical model of the atom
The particle theory of light was also proposed by Ibn al-Haytham, Ibn Sina, Gassendi, and Newton. Those early ideas were founded through abstract, philosophical reasoning rather than experimentation and empirical observation and represented only one line of thought among many. In contrast, certain ideas of Gottfried Wilhelm Leibniz (see "Monadology") contradict to almost everything known in modern physics. In the 19th century, John Dalton, through his work on stoichiometry, concluded that each chemical element was composed of a single, unique type of particle. Dalton and his contemporaries believed those were the fundamental particles of nature and thus named them atoms, after the Greek word "atomos", meaning "indivisible" or "uncut".
History of subatomic physics | Early development
Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center. Ernest Rutherford and his colleagues Hans Geiger and Ernest Marsden came to have doubts about the Thomson model after they encountered difficulties when they tried to build an instrument to measure the charge-to-mass ratio of alpha particles (these are positively-charged particles emitted by certain radioactive substances such as radium). The alpha particles were being scattered by the air in the detection chamber, which made the measurements unreliable.
Atomic theory | Discovery of the nucleus
The concept was formulated in 1904 by German chemist Richard Abegg. Gilbert N. Lewis was one of the first to refer to the concept as "Abegg's rule" when he used it as a basis of argument in a 1916 article to develop his cubical atom theory, which developed into the octet rule. That article helped inspire Linus Pauling to write his 1938 textbook "The Nature of the Chemical Bond".
Abegg's rule | History
Several of these doctrines of atomism are, in some respects, "suggestively similar" to that of Democritus. McEvilley (2002) assumes that such similarities are due to extensive cultural contact and diffusion, probably in either direction. The Nyaya–Vaisesika school developed one of the earliest forms of atomism; scholars date the Nyaya and Vaisesika texts from the 9th to 4th centuries BCE. Vaisesika atomists posited the four elemental atom types, but in Vaisesika physics atoms had 25 different possible qualities, divided between general extensive properties and specific (intensive) properties. The Nyaya–Vaisesika atomists had elaborate theories of how atoms combine. In Vaisesika atomism, atoms first combine in pairs (dyads), and then group into trios of pairs (triads), which are the smallest visible units of matter.
Atomism | Indian atomism
According to Aristotle, Democritus and Leucippus, and many other physicists, proposed the atomic theory, which supposes that everything in the universe is either atoms or voids, specifically to contradict Parmenides' argument. Karl Popper wrote: So what was really new in Parmenides was his axiomatic-deductive method, which Leucippus and Democritus turned into a hypothetical-deductive method, and thus made part of scientific methodology.
Parmenides | Legacy
Angelucci wrote the first version of his theories of matter, energy and life, "The Nature of Infinite Entities" in 1952, based on "research" done earlier in Trenton, including the launching of a giant cluster of weather balloons.
Orfeo Angelucci | Introduction
By 1914, experiments by physicists Ernest Rutherford, Henry Moseley, James Franck and Gustav Hertz had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower-mass electrons. In 1913, Danish physicist Niels Bohr postulated that electrons resided in quantized energy states, with their energies determined by the angular momentum of the electron's orbit about the nucleus. The electrons could move between those states, or orbits, by the emission or absorption of photons of specific frequencies. By means of these quantized orbits, he accurately explained the spectral lines of the hydrogen atom. However, Bohr's model failed to account for the relative intensities of the spectral lines and it was unsuccessful in explaining the spectra of more complex atoms.
Electron | Atomic theory
Despite the importance of the work as the first view of atoms as physically real entities and introduction of a system of chemical symbols, "New System of Chemical Philosophy" devoted almost as much space to the caloric theory as to atomism.
History of chemistry | John Dalton
In 1815, William Prout reported on his observation that the atomic weights of the elements were whole multiples of the atomic weight of hydrogen. He then hypothesized that the hydrogen atom was the fundamental object and that the other elements were a combination of different numbers of hydrogen atoms.
Whole number rule | Prout's hypothesis
With this model Langmuir was able to qualitatively explain the chemical properties of all elements in the periodic table, which were known to largely repeat themselves according to the periodic law. In 1924, Austrian physicist Wolfgang Pauli observed that the shell-like structure of the atom could be explained by a set of four parameters that defined every quantum energy state, as long as each state was occupied by no more than a single electron. This prohibition against more than one electron occupying the same quantum energy state became known as the Pauli exclusion principle. The physical mechanism to explain the fourth parameter, which had two distinct possible values, was provided by the Dutch physicists Samuel Goudsmit and George Uhlenbeck.
Electron | Atomic theory
The basic idea that matter is made up of tiny indivisible particles is very old, appearing in many ancient cultures such as Greece and India. This ancient idea was based in philosophical reasoning rather than scientific reasoning. The word "atom" is derived from the Greek word "atomos", which means "uncuttable".
Atom | In philosophy
The origin of the idea in the Western tradition can be traced to the 5th century BCE starting with the Ancient Greek pre-Socratic philosopher Democritus and his teacher Leucippus, who theorized matter's divisibility beyond what can be perceived by the senses until ultimately ending at an indivisible atom. The Indian philosopher Kanada also proposed an atomistic theory, however there is ambiguity around when this philosopher lived, ranging from sometime between the 6th century to 2nd century BCE. Atomism is explored in Plato's dialogue Timaeus and was also supported by Aristotle. Andrew Pyle gives a lucid account of infinite divisibility in the first few pages of his "Atomism and its Critics".
Infinite divisibility | In philosophy
Lemaître also proposed what later became known as the "Big Bang theory" of the origin of the universe, initially calling it the "hypothesis of the primeval atom".
Georges Lemaître | Introduction
The popular theory of atomic structure at the time of Rutherford's experiment was the "plum pudding model". This model was devised by Lord Kelvin and further developed by J. J. Thomson. Thomson was the scientist who discovered the electron, and that it was a component of every atom. Thomson believed the atom was a sphere of positive charge throughout which the electrons were distributed, a bit like raisins in a Christmas pudding. The existence of protons and neutrons was unknown at this time. They knew atoms were very tiny (Rutherford assumed they were in the order of 10 m in radius).
Geiger–Marsden experiment | Contemporary theories of atomic structure
Kümmel comments: Considering the fact that the CC method was well understood around the late fifties[,] it looks strange that nothing happened with it until 1966, as Jiří Čížek published his first paper on a quantum chemistry problem. He had looked into the 1957 and 1960 papers published in "Nuclear Physics" by Fritz and myself. I always found it quite remarkable that a quantum chemist would open an issue of a nuclear physics journal. I myself at the time had almost given up the CC method as not tractable and, of course, I never looked into the quantum chemistry journals.
Coupled cluster | Historical accounts
German physicist Paul Drude proposed his model in 1900 when it was not clear whether atoms existed, and it was not clear what atoms were on a microscopic scale. The first direct proof of atoms through the computation of the Avogadro number from a microscopic model is due to Albert Einstein, the first modern model of atom structure dates to 1904 and the Rutherford model to 1909. Drude starts from the discovery of electrons in 1897 by J.J. Thomson and assumes as a simplistic model of solids that the bulk of the solid is composed of positively charged scattering centers, and a sea of electrons submerge those scattering centers to make the total solid neutral from a charge perspective.
Drude model | History
The last minuscule particle of matter that could not be subdivided further was termed Parmanu. These philosophers considered the atom to be indestructible and hence eternal. The Buddhists thought atoms to be minute objects unable to be seen to the naked eye that come into being and vanish in an instant. The Vaisheshika school of philosophers believed that an atom was a mere point in space. It was also first to depict relations between motion and force applied. Indian theories about the atom are greatly abstract and enmeshed in philosophy as they were based on logic and not on personal experience or experimentation.
History of physics | India and China
The true beginning of atomic physics is marked by the discovery of spectral lines and attempts to describe the phenomenon, most notably by Joseph von Fraunhofer. The study of these lines led to the Bohr atom model and to the birth of quantum mechanics. In seeking to explain atomic spectra an entirely new mathematical model of matter was revealed. As far as atoms and their electron shells were concerned, not only did this yield a better overall description, i.e. the atomic orbital model, but it also provided a new theoretical basis for chemistry (quantum chemistry) and spectroscopy. Since the Second World War, both theoretical and experimental fields have advanced at a rapid pace.
Atomic physics | History and developments
The first explicitly materialistic system was formed by Leucippus (5th century BC) and his pupil Democritus (460-370 BC) from Thrace. This was the doctrine of atoms - small primary bodies infinite in number, indivisible and imperishable, qualitatively similar, but distinguished by their shapes. Moving eternally through the infinite void, they collide and unite, thus generating objects which differ in accordance with the varieties, in number, size, shape, and arrangement, of the atoms which compose them.
Pre-Socratic philosophy | Atomist school
This principle led to the development of the first atomic bomb during World War II, and the subsequently other nuclear weapons and nuclear reactors.
Lise Meitner | Introduction
This postulate, however, does not apply to the universe's most abundant, and simplest element from the periodic table of elements: hydrogen, with an atomic number of 1. This may be because, in its ionized form, a hydrogen atom becomes a single proton, of which is theorized to have been one of the first major conglomerates of quarks during the initial second of the Universe's inflation period, following the Big Bang. In this period, when inflation of the universe had brought it from an infinitesimal point to about the size of a modern galaxy, temperatures in the particle soup fell from over a trillion degrees to several million degrees.
Oddo–Harkins rule | Exceptions to the rule
In 1924, Louis de Broglie proposed that all moving particles—particularly subatomic particles such as electrons—exhibit a degree of wave-like behavior. Erwin Schrödinger, fascinated by this idea, explored whether or not the movement of an electron in an atom could be better explained as a wave rather than as a particle. Schrödinger's equation, published in 1926, describes an electron as a wave function instead of as a point particle. This approach elegantly predicted many of the spectral phenomena that Bohr's model failed to explain. Although this concept was mathematically convenient, it was difficult to visualize, and faced opposition. One of its critics, Max Born, proposed instead that Schrödinger's wave function described not the electron but rather all its possible states, and thus could be used to calculate the probability of finding an electron at any given location around the nucleus.
Atomic theory | Quantum physical models of the atom
The letter is widely credited with setting in motion the Manhattan Project, the US government effort that built the first atomic bomb. When Szilárd first explained the concept of a nuclear chain reaction to him, Einstein gave the famous reply, "Daran habe ich gar nicht gedacht!" ("I really never thought of that before").
Cutchogue, New York | Albert Einstein and Cutchogue
The most important of all Dalton's investigations are concerned with the atomic theory in chemistry. While his name is inseparably associated with this theory, the origin of Dalton's atomic theory is not fully understood. The theory may have been suggested to him either by researches on ethylene ("olefiant gas") and methane ("carburetted hydrogen") or by analysis of nitrous oxide ("protoxide of azote") and nitrogen dioxide ("deutoxide of azote"), both views resting on the authority of Thomas Thomson. From 1814 to 1819, Irish chemist William Higgins claimed that Dalton had plagiarised his ideas, but Higgins' theory did not address relative atomic mass.
John Dalton | Atomic theory
The theory originally proposed that "all" atomic nuclei are produced by the successive capture of neutrons, one mass unit at a time. However, later study challenged the universality of the successive capture theory. No element was found to have a stable isotope with an atomic mass of five or eight. Physicists soon noticed that these "mass gaps" would hinder the production of elements beyond helium. Just as it's impossible to climb a staircase one step at a time when one of the steps is missing, this discovery meant that the successive capture theory could not account for higher elements. It was eventually recognized that most of the heavy elements observed in the present universe are the result of stellar nucleosynthesis in stars, a theory first suggested by Arthur Stanley Eddington, given credence by Hans Bethe, and quantitatively developed by Fred Hoyle and a number of other scientists.
Alpher–Bethe–Gamow paper | Main shortcoming of the theory
This reconciled the two opposing theories of particle versus wave electrons and the idea of wave–particle duality was introduced. This theory stated that the electron may exhibit the properties of both a wave and a particle. For example, it can be refracted like a wave, and has mass like a particle. A consequence of describing electrons as waveforms is that it is mathematically impossible to simultaneously derive the position and momentum of an electron. This became known as the Heisenberg uncertainty principle after the theoretical physicist Werner Heisenberg, who first described it and published it in 1927. This invalidated Bohr's model, with its neat, clearly defined circular orbits.
Atomic theory | Quantum physical models of the atom
In 1942, his friend and colleague, J. Robert Oppenheimer, held a meeting with several leading theorists at UC Berkeley. The topic of the meeting: develop preliminary plans to design and build a nuclear weapon. Manley, one the attendees, was tasked with learning more about the properties of fast neutrons.
John Henry Manley | Introduction
Throughout the 19th century, chemistry was divided between those who followed the atomic theory of John Dalton and those who did not, such as Wilhelm Ostwald and Ernst Mach. Although such proponents of the atomic theory as Amedeo Avogadro and Ludwig Boltzmann made great advances in explaining the behavior of gases, this dispute was not finally settled until Jean Perrin's experimental investigation of Einstein's atomic explanation of Brownian motion in the first decade of the 20th century. Well before the dispute had been settled, many had already applied the concept of atomism to chemistry. A major example was the ion theory of Svante Arrhenius which anticipated ideas about atomic substructure that did not fully develop until the 20th century.
History of chemistry | 19th century
In 1913, amateur Dutch physicist Antonius van den Broek was the first to propose that the atomic number (nuclear charge) determined the placement of elements in the periodic table. He correctly determined the atomic number of all elements up to atomic number 50 (tin), though he made several errors with heavier elements. However, Van den Broek did not have any method to experimentally verify the atomic numbers of elements; thus, they were still believed to be a consequence of atomic weight, which remained in use in ordering elements. Moseley was determined to test Van den Broek's hypothesis. After a year of investigation of the Fraunhofer lines of various elements, he found a relationship between the X-ray wavelength of an element and its atomic number.
History of the periodic table | Atomic number
The cubical atom was an early atomic model in which electrons were positioned at the eight corners of a cube in a non-polar atom or molecule. This theory was developed in 1902 by Gilbert N. Lewis and published in 1916 in the article "The Atom and the Molecule" and used to account for the phenomenon of valency. Lewis's theory was based on Abegg's rule. It was further developed in 1919 by Irving Langmuir as the cubical octet atom. The figure below shows structural representations for elements of the second row of the periodic table. Although the cubical model of the atom was soon abandoned in favor of the quantum mechanical model based on the Schrödinger equation, and is therefore now principally of historical interest, it represented an important step towards the understanding of the chemical bond.
Cubical atom | Introduction
As to the source of these particles, Thomson believed they emerged from the molecules of gas in the vicinity of the cathode. Thomson imagined the atom as being made up of these corpuscles orbiting in a sea of positive charge; this was his plum pudding model. This model was later proved incorrect when his student Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom.
J. J. Thomson | Conclusions
Atoms were thought to be the smallest possible division of matter until 1897 when J. J. Thomson discovered the electron through his work on cathode rays. A Crookes tube is a sealed glass container in which two electrodes are separated by a vacuum. When a voltage is applied across the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). He concluded that these rays, rather than being a form of light, were composed of very light negatively charged particles he called "corpuscles" (they would later be renamed electrons by other scientists).
Atomic theory | Discovery of subatomic particles
Later this problem was completely solved with the advent of quantum theory.
Johannes Thiele (chemist) | Introduction
Ostwald introduced the word mole into the lexicon of chemistry around 1900. He defined one mole as the molecular weight of a substance in units of mass grams. The concept was linked to the ideal gas, according to Ostwald. Ironically, Ostwald's development of the mole concept was directly related to his philosophical opposition to atomic theory, against which he (along with Ernst Mach) was one of the last holdouts. He explained in a conversation with Arnold Sommerfeld that he was convinced by Jean Perrin's experiments on Brownian Motion. In 1906 Ostwald was elected a member of the International Committee on Atomic Weights.
Wilhelm Ostwald | Atomic theory
Booth were then able to confirm that Bohr was correct. Leo Szilard and Walter Zinn soon confirmed that more than one neutron was released per fission, which made it almost certain that a nuclear chain reaction could be initiated, and therefore that the development of an atomic bomb was a theoretical possibility. There were fears that a German atomic bomb project would develop one first, especially among scientists who were refugees from Nazi Germany and other fascist countries. At the University of Birmingham in Britain, the Australian physicist Mark Oliphant assigned two refugee physicists—Otto Frisch and Rudolf Peierls—the task of investigating the feasibility of an atomic bomb, ironically because their status as enemy aliens precluded their working on secret projects like radar.
Calutron | Origins
Dirac tried to argue that this was due to the electromagnetic interactions with the sea, until Hermann Weyl proved that hole theory was completely symmetric between negative and positive charges. Dirac also predicted a reaction + → + , where an electron and a proton annihilate to give two photons. Robert Oppenheimer and Igor Tamm proved that this would cause ordinary matter to disappear too fast. A year later, in 1931, Dirac modified his theory and postulated the positron, a new particle of the same mass as the electron. The discovery of this particle the next year removed the last two objections to his theory.
Antiparticle | Dirac hole theory