The two simplest kinds of atoms are hydrogen (H),and helium (He), diagramed on Page 1. Hydrogen has one proton in its nucleus and one electron around it. Helium has two protons and hence must have two electrons, since the number of positive and negative charges in a neutral atom must be the same. Because electrons surround an atom, and the nucleus is small and deeply buried, the outer part of the electron cloud is all that another atom "sees." It is the electron cloud that gives each atom its chemical character.

Reactions leading to the making of chemical bonds involve the gain, loss, or sharing of electrons between atoms, as we shall see in subsequent chapters. Since the number of electrons in a neutral atom must equal the number of protons in its nucleus, the number of protons indirectly decides the chemical behavior of the atom. All atoms with the same number of protons are defined as the same chemical element, and the number of protons is its atomic number.

The atomic number sometimes is written as a subscript in front of the symbol of the element, such as H and He. This is convenient but unnecessary, since, for example, every atom with atomic number 2 by definition is called helium and given the symbol He.