Question

In which of the following, elements are not in correct order with respect to the property mentioned in brackets?

• A. \( S < P < N < O \) (Electronegativity)
• B. \( Br < Ge < Ga < Ca \) (Atomic radius)
• C. \( Al < Mg < S < P \) (First ionization enthalpy)
• D. \( Mg < Ca < K < Cs \) (Metallic nature)

Hint:

When evaluating "not in correct order" questions, always check for common anomalies like full-filled/half-filled orbitals in Ionization Enthalpy and specific group vs. period positioning for Electronegativity.

Answer 1

The Correct Option is D

Step 1: Note the four trends we need.
Across a period (left to right): electronegativity rises, atomic radius shrinks, ionization energy rises, metallic nature falls. Down a group the opposite happens.
Step 2: Check the electronegativity option.
The true order is $O>N>S>P$, so written small to big it is $P<S<N<O$. The option gives $S<P<N<O$, which swaps S and P. So this one is wrong, but let us check all to be sure.
Step 3: Check the atomic radius option.
For period 4, radius falls left to right, so $Ca>Ga>Ge>Br$. Written small to big that is $Br<Ge<Ga<Ca$. This matches, so it is correct.
Step 4: Check the ionization energy option.
Half filled $3p^3$ in P and filled $3s^2$ in Mg are extra stable, giving $P>S$ and $Mg>Al$. So $Al<Mg<S<P$ is correct.
Step 5: Check the metallic nature option.
Metallic nature grows down a group and group 1 beats group 2. So $Mg<Ca<K<Cs$ is correct.
Step 6: Pick the odd one.
Only the electronegativity option breaks the rule because P and S are placed the wrong way round. \[ \boxed{S<P<N<O\ \text{is the wrong order}} \]