What is Valency?
The valency of an element is the number of hydrogen atoms that can combine with or replace (either directly or indirectly) one atom of the element. In other words, the valency of an element is the number of electrons an atom of the element uses to combine with atoms of other elements - it is the combining power of an atom of the element. In an atom, the valence electrons are the electrons that can be used in combining with other atoms - these are the electrons in the orbitals of the outermost shell (also called valence shell).
Notice that it is not in all cases that the valency of an atom equals the total number of its valence electrons.
For example, oxygen has six valence electrons, but its valency is 2. Some elements may have more than one combining power (or valency), while others have just one.
For example, H →1; Mg→2; Al→3; C→4; N→3, and 5; P→3 and 5; O→2; S→ 2, 4 and 6; Cl→ 1; and Ne→0.
The valencies of radicals are same as the number of charge they carry.
NH4+ →1; OH- → 1 ; and SO42- → 2.
Note: when two elements combine, or when two oppositely charged radicals combine to form compounds, the molecular formula of the product is simply the interchange of their valencies to obtain the ratio of their combining atoms or groups.
1. The combination of sodium and oxygen. Sodium has valency of 1, while oxygen has valency of 2. By interchanging their valencies to obtain the ratio of their combining atoms, we have Na2O.
2. The combination of NH4+ and SO42-. NH4+ has valency of 1 and SO42- has valency of 2. By interchanging their valencies to obtain the ratio of their combining groups, we have (NH4)2SO4 as the formula of ammonium tetraoxosulphate(VI), the product.
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