What is an Atom ?
Definition of an atom:
Atoms are the smallest particles of an element that have the chemical properties of that element.
Atoms are therefore extremely small (the hydrogen atom is the smallest) and are sometimes described as the "basic building blocks of matter".
This description makes more sense the more chemistry you have studied because a large aspect of the study of chemistry concerns learning about how, why and when certain substances, all of which are made-up of atoms or of atoms that have joined together to form molecules, change in structure and/or behaviour (properties).
What are atoms made of ?
All atoms consist of three particles, called protons, neutrons and electrons.
These are called sub-atomic particles. They are (obviously) much smaller than the atom itself and have the following properties:
Mass (atomic mass unit u)
1/1836 ≈ 0.0005 ≈ zero
≈ means "approximately equal to"
The nucleus of an atom:
Each atom has a nucleus at approximately the centre of its volume consisting of the two types of particles called protons and neutrons. Protons and neutrons are known collectively as nucleons.
Neutrons have no charge and protons have a charge of +1 (that is, they are positively charged).
Therefore the nucleus of an atom is positively charged.
Or, if describing more than one atom, one would say that the nuclei (plural) of the atoms are positively charged.
The nucleus is tiny compared with the total volume of the atom.
That is, it occupies a very small space compared with the volume around it, BUT the nucleus accounts for most of the mass ('mass' is the correct word, but sometimes it is colloquially called 'weight') of the atom. That is clear from the relative masses of protons, neutrons and electrons listed in the table above.
Outside the nucleus of an atom:
The volume of the rest of the atom (surrounding the nucleus) is occupied by the other type of sub-atomic particle, which is called an electron, or rather (in all cases except for hydrogen) electrons because there are many electrons per atom.
Electrons have hardly any mass.
Each electron has a charge of -1 (that is, they are negatively charged).
Electrons are distributed around the volume of the atom of which they are a part in 'shells', 'orbits' or 'energy levels', which are all different terms used to describe the electronic structure of atoms, an important topic necessary for understanding most aspects of chemistry. In the simplest terms electrons may be said to 'move around the nucleus' and are sometimes drawn as if circling or rotating around the atom at a distance or sets of distances from the nucleus.
Charge of an atom:
Each atom has an equal number of protons (+1 charge) and electrons (-1 charge), therefore the overall charge of the atom is 0, or 'neutral'.
If one or more electrons are removed then the particle would be positively charged and if one or more additional electrons joined the particle then it would be negatively charged ... but in either case it would then no-longer be referred to as an 'atom'.
Atoms are always neutral (no charge).
How to describe specific atoms:
Atoms can be described by just a few pieces of information.
Each element also has an Atomic Number and a Mass Number. Together, these describe the atoms of that particular element. It is possible to describe all known elements in a single table - which is called the Periodic Table. In it each element is represented by a square or rectangular area containing the symbol of the element in the centre, its atomic number in one corner and its mass number in the corner below (or above*).
Note: The periodic table is not just a page of summary information about atoms in boxes - the layout of the table is also important and informative, but that is another topic.
The example on the right shows the information about sodium as shown in the Periodic Table. This states that sodium has an atomic number of 11 and a mass number of 23.
The Atomic Number of an atom:
... is the number of protons in the atom. It is sometimes represented by the symbol 'Z'.
The Atomic Number (Z) is sometimes called the 'Proton Number'.
The Mass Number of an atom:
... is the total number of protons and neutrons in the atom. It is sometimes represented by the symbol 'A'.
The Mass Number (A) is sometimes called the 'Nucleon Number'.
Continuing with the example of sodium:
The atomic number of sodium is 11 so there are 11 protons in an atom of sodium. The mass number of sodium is 23 and its atomic number is 11 so we can work-out that there are:
23 - 11 = 12 neutrons in a sodium atom. We know that there are 11 electrons in an atom of sodium because atoms have zero (= 'neutral') charge, so the number of electrons must be equal to the number of protons.
What defines a specific type of Atom ?
Continuing use of sodium as an example, what makes a sodium atom, a sodium atom ?
Could another type of atom just happen to have 11 protons, 11 electrons and 12 neutrons ? No.
The number of protons = the 'Atomic Number', which is sometimes referred to as 'Z',
defines the type of atom, i.e. which element it is an atom of.
As indicated in the List of Chemical Elements, each atomic number corresponds to one single specific element.
What about the number of neutrons in atoms ?
Some elements have atoms that can include more than one number of neutrons in their nuclei.
Isotopes are elements that have the same atomic number (which - as stated above - is what makes them the same element) but different Mass Numbers (A), due to different numbers of neutrons in their nuclei.
For more about this see What is an isotope ?
See also more words related to the word "atom" that may be useful in the study of chemistry and other physical science subjects (and for general information) on the page What is Atomic?.