Question

What are the valencies of Sulphur in $S{O_2}$ and $S{O_3}$.

Answer 1

Hint:To answer this question, you must recall the structure and electronic arrangement of the compounds given in the question. Valency of an element describes its combining capacity.

Complete answer:
The valency of an atom is the number of electrons present in the valence shell of the atom. These electrons are the most loosely held electrons in the atom and thus determine the properties of the element.
In a compound, the valency of an atom is the number of bonds it forms or the number of electrons involved in bonding, i.e., lost, shared, or gained. We know that the atomic number of Sulphur is 16 and the number of electrons in its valence shell is 6. So, generally, the valency of Sulphur must be 2, but it can show multiple valencies as well.
In Sulphur dioxide, Sulphur is bonded to 2 oxygen atoms. Oxygen is more electronegative than Sulphur and thus, shows a fixed valency of 2. As a result each oxygen forms two bonds with the Sulphur atom making its valency 4.
In Sulphur trioxide, Sulphur is bonded to 3 oxygen atoms. Oxygen is more electronegative than Sulphur and thus, shows a fixed valency of 2. As a result each oxygen forms two bonds with the Sulphur atom making its valency 6.

Note:
Each atom aims to attain a stable electronic configuration which is similar to that of a noble gas. This electronic configuration is attained by losing, gaining or sharing electrons. Valency does not always necessarily represent the number of valence electrons. For valence electrons less than 4, the valency is equal to the number of valence electrons and for valence electrons more than 4, the valency is given by subtracting the number of electrons from 8.