Protons and neutrons are packed in a very small nucleus buried in a cloud of electrons. Though the nucleus is not unimportant, the electrons determine how an element behaves chemically.

The electrons are arranged about the nucleus in specific regions called shells. The electrons of lower energy are held nearer the nucleus, and those of higher energy are placed farther away on the outer surface of the atom. The electrons in the highest energy shell, called the valence electrons, are the ones that most directly determine the chemistry of the element. Each shell is further broken down into subshells ( s, p, d, and f), and each subshell is made up of 1, 3, 5, or 7 orbitals, which contain the electrons. Each orbital can contain 1 or 2 electrons, or it can be empty. Elements in the same vertical group in the periodic table all have their highest energy electrons arranged in the same way, and they are, therefore, chemically similar.