Valency is the combining capacity of an element. It can be treated as the number of electrons lost, gained or shared by an atom in a chemical combination.
The following are the steps to write chemical formulae from valency.
•First write the element with lower electronegativity.
•Exchange the valency of each element and write as suffix.
•Divide the suffix with common factor.
•If the suffix is 1, need not be written.
a) Ba is written first since it has lower electronegativity. The valences are exchanged and put as suffix, so Ba gets chlorine valency 1 as suffix and Cl gets barium valency 2 as suffix. Division by common factor 1 yields same. the suffix one is not needed. so, its simply written as BaCl2.
b) Zn is written first since it has lower electronegativity. The valences are
exchanged and put as suffix, so Zn gets oxygen valency 2 as suffix and O gets zinc valency 2 as suffix. Division by common factor 2 yields Zn1O1. The suffix one is not needed. so, its simply written as ZnO.
We know that the valency of O is 2, as its given. Let’s consider the valency of Ca is x. So, by following above rules, the chemical formula should be, Ca2Ox. But the formula of calcium oxide is CaO which can be written as Ca1O1. So, the suffix are divided by 2 to get Ca1Ox/2 . Comparing Ca1Ox/2 with Ca1O1 , we get that
x/2 = 1
x = 2.
Hence valency of Ca is 2.
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Some elements andKerala Board Chemistry Part I