JEE Chemistry 2026 Question with Solution

Given: The trend in first ionization enthalpies of selected elements of the $3^{\text{rd}}$ period is to be identified.

Find: The correct increasing order among $Al$, $Si$, $S$, $P$ and $Cl$.

Ionization enthalpy generally increases across a period. However, there are important exceptions due to the extra stability of filled or half-filled subshells and the penetration difference between $s$- and $p$-electrons.

From the solution text, the relevant trend across the $3^{\text{rd}}$ period is:

This indicates two exceptions:

1. $Mg > Al$ because $3s^2$ is more stable than $3p^1$.
2. $P > S$ because $P(3p^3)$ has a half-filled stable configuration, whereas $S(3p^4)$ has one paired electron.

Therefore, among the asked elements:

So the correct option is A.

Given: Ionization enthalpy usually increases from left to right across a period.

Find: Which option correctly accounts for the exception in the $3^{\text{rd}}$ period.

A quick check is to compare the nearby elements and remember the exception: half-filled configuration wins. Thus $P(3p^3)$ has higher ionization enthalpy than $S(3p^4)$.

Now move left to right overall: $Al$ is lower than $Si$, and $Cl$ is highest among the given elements. Therefore the increasing order becomes:

Hence, the correct option is A.