The ability of an atom to combine with other atoms depends on the number of electrons in the outer shells of the atoms. Some elements, the so-called noble gases, have complete outer shells and do not share electrons. Many elements will share electrons with other elements, such that each element completes its outer electron shell capacity.

A shared electron pair is called a covalent bond. The number of covalent bonds that each element can form is called its valence.

The CHNOPS atoms are shown above. Each valence position is represented by a stick protruding from the atom.