Summary

The subatomic particles of most interest to a chemist are electrons, protons, and neutrons. Electrons have a relative charge of - 1 and a relative mass of 5.45 X 10^-4 amu. Protons have a relative charge of + 1 and a relative mass of 1.007 amu. Neutrons have no charge and a relative mass of 1.008 amu. Each atom is a mix of these three particles. The atomic number of an atom is equal to the number of protons in the nucleus of that atom and also to the number of electrons outside the nucleus. Most of the mass of an atom is concentrated in the nucleus, where neutrons and protons are found. The total number of particles in the nucleus (neutrons plus protons) is equal to the mass number of the atom. Atoms of the same element always contain the same number of protons but can differ slightly in the number of neutrons. This variation explains the existence of isotopes, which are atoms of the same element that vary slightly in mass. Rutherford's gold-foil experiment, carried out in 1911, confirmed the existence of the nucleus.

The atomic weight of an element is the average of the relative masses of the naturally occurring atoms of that element. To three significant figures, one mole of an element contains 6.02 X 10²³ atoms of that element. The mole is a counting unit; a mole is 6.02 X 10²³ things. One mole of an element has a mass in grams numerically equal to the atomic weight of the element.

The nuclei of some atoms decompose radioactively to produce alpha particles, beta particles, or gamma rays. These reactions can be shown by equations. The rate of decay of radioactive isotopes is measured in half-lives. Radioactivity has important applications in the health and biological sciences. Nuclear reactions can be used to produce energy.