Valency : Valency

The valency of an element is measured by the number of hydrogen atoms which unite with one atom of that element. Chlorine, oxygen, nitrogen, and carbon are uni-, bi-, ter-, and quadri-valent respectively.

Univalent chlorine may be used instead of hydrogen in determining valencies, which when so found are the same as those referred to hydrogen, but quinque- and sexivalent elements are now included:

Cl₂O (Chlorine monoxide), Cl₃N (Nitrogen trichloride), Cl₄C (Carbon tetrachloride), Cl₅P (Phosphorus pentachloride), Cl₆W (Tungsten hexachloride

In chlorine monoxide, Cl₂O, and calcium chloride, CaCl₂, oxygen and calcium are bivalent. When calcium and oxygen combine, we should therefore expect them to do so atom for atom, since each of the combining atoms has a valency of two units. This is the case; calcium oxide, quicklime, has the formula CaO. Calcium can also displace two atoms of hydrogen from hydrogen chloride: 2HCl + Ca = CaCl₂ + H₂. If chlorine is passed over strongly heated lime, one atom of oxygen is displaced by two of chlorine: 2CaO + 2Cl₂ = 2CaCl₂ + O₂.

There is a close relation between the atomic weight and equivalent of an element and its valency. The equivalent is the weight of an element which combines with or displaces unit weight of hydrogen. The valency is the number of unit weights (atoms) of hydrogen which combine with, or are displaced by, one atomic weight of the element, hence:

Atomic weight = Equivalent x Valency

or Valency = Atomic weight/Equivalent.