A: I.E. of 5d elements > 4d elements in general.
R: It is due to lanthanoid contraction.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To analyze the given assertion and reason, we will break down the concepts step by step.

### Step 1: Understand the Assertion
The assertion states that the ionization energy (I.E.) of 5d elements is greater than that of 4d elements in general.

**Explanation:**
- Ionization energy is the energy required to remove an electron from an atom in its gaseous state.
- The 5d block elements are transition metals that have their outermost electrons in the 5d subshell, while the 4d block elements have their outermost electrons in the 4d subshell.

### Step 2: Understand the Reason
The reason provided is that this trend is due to lanthanoid contraction.

**Explanation:**
- Lanthanoid contraction refers to the phenomenon where the atomic and ionic sizes of the lanthanide series elements decrease with increasing atomic number due to the poor shielding effect of the f-orbitals.
- This contraction affects the 5d elements because the increased nuclear charge (more protons) in these elements leads to a smaller atomic size compared to the 4d elements.

### Step 3: Analyze the Relationship Between Assertion and Reason
Now, we need to see if the reason provided correctly explains the assertion.

- Due to lanthanoid contraction, the atomic size of 5d elements is smaller than that of 4d elements.
- A smaller atomic size means that the outer electrons are closer to the nucleus, which increases the effective nuclear charge experienced by these electrons.
- As a result, the outer electrons in 5d elements are held more tightly by the nucleus, leading to higher ionization energy compared to 4d elements.

### Conclusion
Both the assertion and reason are correct. The reason (lanthanoid contraction) accurately explains why the ionization energy of 5d elements is greater than that of 4d elements.

### Final Answer
Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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