Question

Exception of octet rule is?

• A. $\mathrm{K}$
• B. $\mathrm{Ca}$
• C. $\mathrm{N}$
• D. $\mathrm{He}$

Answer 1

The correct option is D

$\mathrm{He}$

The octet rule asserts that atoms with atomic numbers less than 20 tend to combine in such a way that their valence shells each have eight electrons, giving them the same electrical configuration as a noble gas.

The rule applies to all main-group elements, including Carbon ($C$), Nitrogen($N$), Oxygen ($O$), and halogens, as well as metals like Sodium ($Na$) and Magnesium ($Mg$).

Molecules with an odd number of electrons on the central atom, or molecules with more or less electrons than eight in their valence shell, are the general exceptions to the octet rule.

Explanation for correct option

(D) Helium $\left(\mathrm{He}\right)$ is an exception to the Octet Rule.
Electronic configuration of Helium is $1{\mathrm{s}}^2$, So, based on the arrangement, we can observe that only two electrons are present in the outermost orbital of Helium ($He$), but it is still stable, indicating that it is an exception.

Explanation for incorrect options

(a) Potassium $\left(\mathrm{K}\right)$ follows octet rule as its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^1$and it can attain noble configuration by losing one electron.

(b) Calcium $\left(\mathrm{Ca}\right)$ follows octet rule as its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^{6}4{\mathrm{s}}^2$and it can attain noble configuration by losing two electrons.

(c) Nitrogen $\left(\mathrm{N}\right)$ follows octet rule as its electronic configuration is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^5$ and it can attain noble configuration by gaining one electron.

Thus, option D is the correct answer.