The atomic number is the number of protons contained in an atom's nucleus. When an atom is in its normal state (that is, when it is not ionized), the number of its planetary electrons equals the atomic number. The positive charge of the nucleus is thus equal to the negative charge of the planetary electrons, so that the atom as a whole is electrically neutral. All atoms of the same element have the same atomic number.

The atomic weight of an atom is a relative figure that is compared to the standard figure assigned to the carbon 12 atom, and is approximately equal to the number of protons and neutrons in the atom's nucleus. Chemists and physicists sometimes use the term atomic mass when speaking of specific atoms, preferring to use atomic weight to mean the average weight of all the atoms of an element. (As explained later under Isotopes, atoms of the same element can have different numbers of neutrons and, therefore, different weights.)

There are several methods of determining atomic weights. One of the oldest (and most accurate) methods is based on the long-established fact that when chemical elements unite to form compounds, they do so in fixed proportions. For example, careful weighing has shown that when silver reacts with chlorine to form silver chloride, the two elements always combine in the ratio of 0.32867 gram of chlorine for each gram of silver. Since a silver chloride molecule consists of one atom of chlorine and one atom of silver, the ratio of the atomic weights of chlorine and silver must be 0.32867 gram 1 gram, or 0.32867. Therefore, if the atomic weight of either chlorine or silver is known (from a weight ratio relating either of these elements to carbon, for example), the atomic weight of the other element can be readily calculated.

The mass spectrometer, an instrument in which beams of ions (electrically charged atoms or molecules) are deflected by a magnetic field, is also used to determine atomic weights. The masses of the ions in a beam can be calculated from the amount of deflection.

Most chemical elements, as found in nature, are made up of a mixture of atoms of different atomic weights. Atoms with the same atomic number but different atomic weights are called isotopes. Lithium, for example, has two isotopes: lithium 6, with an atomic weight of 6.015; and lithium 7, with an atomic weight of 7.016. Since there are about 12 times as many atoms of lithium 7 found in nature as atoms of lithium 6, the atomic weight of the element lithium is calculated to be 6.939.

The isotope carbon 12, used as the reference standard for the atomic-weight scale, constitutes almost 99 per cent of the carbon found in nature. (Carbon 13, a stable isotope, and carbon 14, a radioactive isotope, constitute the remainder.) Some elements, such as aluminum, phosphorus, and iodine, possess no natural isotopes; they are made up of only one kind of atom.

Atoms that have the same approximate atomic weight but different atomic numbers are called isobars. Neptunium 239 and plutonium 239, for example, are isobars—both have 239 nucleons but in neptunium 93 are protons and in plutonium 94 are protons.

The electronic structure of atoms can be given in terms of a shell model. Each shell is a group of electrons all roughly the same distance from the nucleus. The number of electrons per shell—as well as the total number of electrons—is the same for all the atoms of a given element, provided that each atom is in its normal state. In the atoms now known, the number of electron shells ranges from one to seven. These shells are designated by the letters K, L, M, N, O, P, and Q, in order of their distance from the nucleus.

An atom of chlorine, for example, has 17 electrons in three shells—2 in the K, or innermost, shell; 8 in the L, or second, shell; and 7 in the M shell. A potassium atom, with 19 electrons, has four shells, with the electrons arranged 2, 8, 8, 1.

The chemical properties of the various elements are determined primarily by the number of electrons in the outermost shells of their atoms. For example, the atoms of the metals lithium, sodium, and potassium, which have similar properties, each have a single electron in their outermost shell. It is for this reason that these elements are grouped together in the Periodic Table, a chart that arranges elements according to their properties.

When atoms take part in a chemical reaction, the electrons in their outermost shells are rearranged. An atom with a single electron in this shell tends to lose the electron. An atom with many electrons in its outermost shell (such as chlorine, with its 7), tends to capture one or more additional electrons until it has a number in this shell that represents a stable arrangement. Groups of 2, 8, 18, and 32 electrons are known to represent highly stable arrangements, and these numbers represent the maximum number of electrons that can exist in the K, L, M, and N shells, respectively, according to quantum mechanics.

Valence is the ability of an atom to gain or lose electrons, or to share them with another atom. An atom displays its valence when it combines with an atom of another element in a chemical reaction.