Question

The covalency of oxygen cannot exceed $2$ , unlike Sulphur which can show a $+ 4$ or $+ 6$ because:
(A) oxygen atom does not have d-orbitals
(B) oxygen atom has two unpaired electrons in its valence shell
(C) oxygen can form a double bond with another oxygen atom
(D) electrons of oxygen atom cannot be promoted to d-orbitals due to its small size

Answer 1

Atoms form covalent bonds by sharing of unpaired electrons present in their valence shell. In some elements electrons from the inner orbitals can be excited to outer vacant orbitals to show extended valency.

Complete step by step answer: 1) We can write the electronic configuration of both oxygen $\left( O \right)$ with atomic number eight and Sulphur $\left( S \right)$ with atomic number sixteen.
2) The electronic configuration of oxygen is $1{s^2}2{s^2}2{p^4}$ and the electronic configuration of Sulphur is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^4}$
3) $2{\text{d}}$ orbitals don’t exist but $3{\text{d}}$ orbitals are possible. Hence, Sulphur has vacant d orbitals which can be used to expand the valency by the Sulphur atoms and form covalent bonds. But oxygen doesn’t have d orbitals thus cannot expand its valency. Thus, Sulphur shows valencies of $+ 4$ and $+ 6$ but oxygen only shows $- 2$ valency.
So, the correct option is ‘A’ i.e. oxygen atom does not have d-orbitals.

Additional information: Noble gases have the most stable electronic configuration that is why they don’t react with other elements. All the other elements tend to attain eight electrons in their valence shell to have noble gas configuration. This is called the octet rule. This is the reason for the bond formation between different atoms. But this octet rule can be ‘expanded’ in some cases by using the d-orbitals present in the outer shells. Sulfur, phosphorus, silicon, and chlorine are some of the elements that show expansion in octet or expanded valency. Examples of molecules where the elements show deviation from octet rule and are surrounded by more than eight electrons around the central atom are: Phosphorus pentachloride $(PC{l_5})$ , sulfur hexafluoride $(S{F_6})$ and chlorine trifluoride $Cl{F_3}$.

Note: When there is a vacant d orbital the paired electrons in the p orbital tend to excite and shift in the next d orbitals to form the covalent bonds with other atoms. The oxygen $\left( O \right)$ atom is in period $2$ in which d orbitals are absent. It should be remembered that $1{\text{p}}$ , $2{\text{d}}$ and $3{\text{f}}$ orbitals don’t exist.