What is the valency ? write the valencies of the calcium, iron, hydroxyh ,nitrate

(a):Valency is the measure of the capacity of an atom of a single element to react and combine with particular numbers of atoms of another element.

(b):

Valency of calcium:+2

Valency of Iron:Iron exhibits 2 valence states of +2 and +3.

Valency of hydroxl:-1

Valency of Nitrate:-1

Explanation:

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your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.