Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R)
Assertion (A): The first ionization enthalpy of $3d$ series elements is more than that of group 2 metals
Reason (R): In $3d$ series of elements successive filling of d-orbitals takes place
In the light of the above statements, choose the correct answer from the options given below :

• A. Both $( A )$ and $( R )$ are true and $( R )$ is the correct explanation of $( A )$
• B. (A) is true but (R) is false
• C. (A) is false but (R) is true
• D. Both $( A )$ and $( R )$ are true but $( R )$ is not the correct explanation of $( A )$

Hint:

In transition metals, the ionization energies are influenced by the filling of d-orbitals, leading to variations from what is observed in s-block elements.

Answer 1

The Correct Option is C

Let's analyze the given statements.

1. Assertion (A): The first ionization enthalpy of $3d$ series elements is more than that of group 2 metals.
2. Reason (R): In the $3d$ series of elements successive filling of d-orbitals takes place.

Explanation:

Ionization enthalpy is the energy required to remove an electron from an atom in the gaseous state. For the $3d$ series of transition metals, the first ionization enthalpy is generally less than the group 2 (alkaline earth) metals.

• In the $3d$ series, the electrons are added to the inner 3d subshell which results in a poor shielding effect. This increases the effective nuclear charge experienced by the outer electrons, leading to higher energy levels but overall the ionization energy remains lower compared to group 2 elements.
• For group 2 elements, as we move down the group, the atomic size increases, but since there is no filling of an inner d-orbital to obscure nuclear charge, their ionization enthalpy is higher compared to the beginning of the $3d$ series.

Therefore, the assertion (A) is incorrect.

Now, considering the reason (R), successive filling of the d-orbitals does indeed occur as we move across the $3d$ series. This statement is factually correct but it doesn't adequately explain why the $3d$ series elements actually have lower ionization enthalpy than group 2 metals.

Conclusion: The correct answer, as provided, is "(A) is false but (R) is true" because the reason is true on its own but it doesn't justify the assertion.