The atom is the basic building block of chemistry. In the simplest terms, it consists of a single positively-charged nucleus, containing Z protons and zero or more neutrons, surrounded by a cloud of electrons. The number Z, known as the atomic number, characterizes the atom uniquely as being that of a particular element. In the neutral atom, the number of electrons is equal to the number of protons; however, electrons may be removed or acquired to form a positively- or negatively-charged ion of the same element.
The chemical activity of atoms is determined almost entirely by electronic structure. The energy required to add or remove electrons from the neutral atom determined the ability to form ions and to take part in the simplest kind of chemical bonding: ionic bonding. Many compounds have molecules in which the atoms are bound together by covalent bonding in which electrons are shared.
The chemical physics of the atom is determined theoretically by quantum mechanics and may be investigated experimentally by Spectroscopy. The electronic structure of atoms so determined is explained well by a simple model in which the electrons are located within a series of shells. Each electron is then identified by a set of four quantum numbers which determine it energy, orbital angular momentum, and spin angular momentum [Atkins (1990)].
The mass of the atom is determined by the total number of nucleons (protons and neutrons) and electrons. Many exist in several forms, known as isotopes, which differ in the number of neutrons present. Unstable isotopes may decay through radioactive nuclear reactions in which elementary particles may be emitted from the nucleus thereby transforming the atoms of one element into those of another.
Atkins, P. W. (1990) Physical Chemistry, 4th ed., Chapter 13, Oxford University Press, Oxford.