The elements are arranged in columns in the periodic table, I-VIII, which represents the number of electrons in their outermost shells. The transition elements have partly filled d or p orbitals. Atoms with similar valence tend to behave similarly, and to substitute for each other in minerals.
Metals have low electronegativity and are electron donors.
They tend to become cations. Nonmetals have high values of electronegativity and are electron receptors, and tend to be anions. The difference in electronegativity can be used to estimate the 'bond character', which can be thought of as the degree of ionic and covalent character to a particular atomic bond.