Physical Science for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide
Chemistry is the study of elements and the compounds they form. Matter can take the form of an element, a compound, or a mixture.
An element is the basic form of matter, incapable of being decomposed by chemical means into simpler substances. Each element has distinct chemical and physical characteristics. Hydrogen (H), oxygen (O), and carbon (C) are elements.
A compound is a combination of two or more elements chemically combined in a specific proportion. Compounds can be separated by chemical means, and are represented by chemical formulas that include the symbols of all the elements present. Examples of familiar compounds are water (H2O) and table salt (NaCl). In order to be considered organic, a compound must contain carbon.
A mixture is a combination of two or more substances that are not chemically combined. Dissolving salt in water results in a mixture. The two compounds don't react with each other and can be separated by physical means—in this case, heating the water so it evaporates, leaving the salt behind.
All matter is made up of atoms. The following terms are used to define atomic structure:
- Atom: the smallest unit of an element that retains all of the element's chemical properties. An atom is composed of three primary particles: electrons, protons, and neutrons.
- Electron: found outside the nucleus (the center of an atom), it has a negligible mass and a charge of –1.
- Proton: found in the nucleus, it has a mass of 1 amu (atomic mass unit) and a charge of +1.
- Neutron: found in the nucleus, it has a mass of 1 amu and no charge.
An atom contains an identical number of protons and electrons, making it electrically neutral.
Atoms of the same element generally have the same properties, unless they are isotopes, which can behave differently. Atoms of different elements have different properties and different masses. Atoms of elements combine in simple whole number ratios.
The periodic table (next page) lists all of the known elements according to their atomic numbers.
Atomic number is the number of protons in the atom. The atomic number determines the element.
Mass number is the total number of protons and neutrons in one atom of an element. Mass number can vary because the number of neutrons in an atom can change.
The horizontal rows of elements in the periodic table are called periods. There are seven periods in all. Moving from left to right across a period, the atomic number increases by one from one element to the next. Each successive element has one more electron in its outer shell. All elements in the same period have the same number of shells.
The vertical columns of elements in the periodic table are groups. Elements in the same group have the same number of electrons in their outer shell. They therefore have similar chemical properties.
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