Explain the term Valency of an element. How does it vary in a period and in a group in the periodic table? | Class XI

Ans: The chemical properties of elements depends upon the number of electrons in the outer most shell of an atom. These electrons are called valence electrons and thus determine the valency of the atom or element.
In representative elements, the valency is generally equal to either n or (8 — n), where n is number of valence electrons in the atom.
In a period valence electrons increase from 0 to 8 on moving from left to right. The valency of an element w.r.t. H and Cl increases from 1 to 4 and then decreases to zero. However w.r.t. oxygen, valency increases from 1 to 7 and then becomes zero in noble gases.
In a group, the number of valence electrons remains the same and therefore all elements in a group exhibit same valency, e.g., all elements of group I have valency one and those of group 2 have valency two. However, the transition elements exhibit variable valency.

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