The valence of an atom is related to the ability of the atom to enter into chemical combination with other elements and is often determined by the number of electrons in the outermost sp level. Some examples are:
Mg:
1s 22s 22p 6 3s 2
valence = 2
Mg:
1s 22s 22p 6 3s 23p1
valence = 3
Mg:
1s 22s 22p 63s 23p 63d 10 4s 24p 2
valence = 4
Valence also depends on the nature of the chemical reaction. Phosphorus has a valence of five when it combines with oxygen. But the valence of phosphorus is only three — the electrons in the 3p level — when it reacts with hydrogen. Manganese may have a valence of 2, 3, 4, 6, or 7.