The combing capacity of an element is known as valency. It depends on the no. of electrons needs to an atom in order to get stability
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Valency is the combining capacity of an element. For a non-metal valency= 8 - Valency electron.(no. of electrons in the outermost shell of the atom. For example: Valency of Oxygen is 2 that is 8-6(valence electons) If Oxygen gets two more electons then is gains stability ie. forms its octet. Some transition metals have variable valency. ex:Fe. But generally valency of metal=valence electrons ex: sodiun(Na) has a valency of 1 ie. If it loses it one electron it gains stability.