Monday, June 20, 2011


The valency of an element may be defined as the combining capacity of element. It is generally expressed in terms of number of hydrogen atoms or the number of chlorine atoms or double the number of oxygen atoms that combine with an atom of the element or replace it. The electrons present in the outermost shell are called Valence Electrons and these electrons determine the valency of the atom.
Variation of Valency
(a).  Variation along period: – Valency of normal elements i.e. s and p-block elements first increases from 1 to 4 and then decreases from 3 to zero. But d-block elements show variable valencies. f-block elements exhibit only electropositive valency, the most common being 3.
(b).  Variation down the group :– On moving down a group, the number of valence electrons remains same and therefore, all the elements in a group exhibit same valency. For example, all the elements of group 1 have valency equal to one and those of group 2 have valency equal to two.
CTM:–  Valency can be determined as follows:-
If no. of electrons in valence shell is 1 to 4 then valency of atom is equal to no. of electrons in valence shell. If no. of electrons in valence shell is 5 to 8, then valency is equal to valence electron -8

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