Question

How to know valencies of elements of variable valencies in a compound? Ex- lead oxide. Here what should we take the valency of lead?

Answer 1

The electrons which are present in the outermost shell of an atom are known as valence electrons. These are the electrons that participate in molecule or chemical bond formation. The capacity of an element to combine with other atoms to form molecules or chemical compounds is known as valency.

Complete step-by-step answer: Valency of an element can be calculated using two simple steps. Let us take an example to lead. Lead is a heavy metal having atomic number 82.
1. First, we must write down the electronic configuration of the element. This will help us figure out how many electrons are there in the valence shell.
Electronic configuration of Pb: $[Xe]4{{f}^{14}}5{{d}^{10}}6{{s}^{2}}6{{p}^{2}}$.
From the electronic configuration, we can see that the valence shell of lead has 4 electrons.
2. If the number of electrons in the outermost shell is less than or equal to 4, the valency is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell is more than, the valency is equal to 8 - (the number of electrons in the outermost shell).
Since lead has four electrons in its valence shell, one would assume that its valency is 4.
But this is not the case as lead shows different valencies in different compounds. For example, in lead (ii) oxide (PbO), the valency of lead is +2, whereas in lead dioxide $(Pb{{O}_{2}})$, lead shows a valency of +4
This is because of the concept of variable valency. An element is said to exhibit variable valency, when it shows different valency under different chemical reaction conditions.
Now, metals form positively charged ions by donating electrons from their valence shell and it is known as electropositive valency. But sometimes, in transition elements, the electrons are lost from the penultimate shell as well. These are the elements which exhibit variable valency.
So, to find out the valency of an element having variable valencies in a compound, we need to know the proper molecular and chemical formula of the compound, the oxidation state of the element in the compound, etc. Careful observation of the combining of the element and the octet rule can help in determining the valency.

Note: It should be noted that the nomenclature can also help in identifying valency. When we have to indicate a lower valency, the suffix "ous" is used. (Ferrous oxide, FeO, valency=2). To indicate a higher valency, "ic" suffix is used. (Ferric oxide, $F{{e}_{2}}{{O}_{3}}$,valency=3)