Common Valences
For elements in the main groups of the periodic table, the valence can vary between one and seven, but usually these elements form a number of valence bonds between one and four. The number of bonds formed by a given element was originally thought to be a fixed chemical property. In fact, in most cases this is not true. For example, phosphorus often has a valence of three, but can also have other valences.
Nevertheless, many elements have a common valence related to their position in the periodic table, following the octet rule. Elements in the main groups 1 (alkali metals) and 17 (halogens) commonly have a valence of 1; elements in groups 2 (alkaline earth metals) and 16 (chalcogens) valence 2; elements in groups 13 (boron group) and 15 (nitrogen group) valence 3; elements in group 14 (carbon group) valence 4.
Polyvalence or multivalence refers to species that are not restricted to a distinct number of valence bonds. (Species with only one valence are univalent (monovalent)). For example, the Cs+ cation is a univalent or monovalent cation, whereas the Ca2+ cation is a divalent or polyvalent or multivalent cation and the Fe3+ cation is a trivalent or polyvalent or multivalent cation. As a result, examples of polyvalent cations include the Ca2+ cation and the Fe3+ cation.
Read more about this topic: Periodic Table (valence)
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