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Chemistry Question 59 – JEE-MAIN 2026

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Pairs of elements with the same number of electrons in their respective 4f orbital are \ [Atomic number: Eu-63, Gd-64, Dy-66, Ho-67, Tm-69, Yb-70, Lu-71, Hf-72] \ A. (Eu and Gd) \ B. (Dy and Ho) \ C. (Yb and Hf) \ D. (Lu and Tm) \ Choose the correct answer from the options given below:

Recall the general electronic configuration of lanthanides and how exceptions occur for stability (half-filled or fully-filled f-orbitals, or 5d1 configuration).

Step 1: Determine the electronic configuration for each element✦ Active

We need to find the number of electrons in the 4f orbital for each given element. The general electronic configuration for lanthanides is [Xe]4fn5d0 or 5d16s2. Exceptions occur for stability (half-filled 4f7 or fully-filled 4f14).

$\text{Eu (Z=63): } [Xe] 4f^7 6s^2 \implies 7 \text{ 4f electrons} \ \text{Gd (Z=64): } [Xe] 4f^7 5d^1 6s^2 \implies 7 \text{ 4f electrons (exception for stability)} \ \text{Dy (Z=66): } [Xe] 4f^{10} 6s^2 \implies 10 \text{ 4f electrons} \ \text{Ho (Z=67): } [Xe] 4f^{11} 6s^2 \implies 11 \text{ 4f electrons} \ \text{Tm (Z=69): } [Xe] 4f^{13} 6s^2 \implies 13 \text{ 4f electrons} \ \text{Yb (Z=70): } [Xe] 4f^{14} 6s^2 \implies 14 \text{ 4f electrons} \ \text{Lu (Z=71): } [Xe] 4f^{14} 5d^1 6s^2 \implies 14 \text{ 4f electrons (exception for stability)} \ \text{Hf (Z=72): } [Xe] 4f^{14} 5d^2 6s^2 \implies 14 \text{ 4f electrons (Hf is a d-block element, 4f is filled)}
Step 2: Evaluate each given pair○ Expand

Now, let's check which pairs have the same number of 4f electrons:

\text{A. (Eu and Gd): Eu (7 4f), Gd (7 4f) } \implies \text{Match} \ \text{B. (Dy and Ho): Dy (10 4f), Ho (11 4f) } \implies \text{No Match} \ \text{C. (Yb and Hf): Yb (14 4f), Hf (14 4f) } \implies \text{Match} \ \text{D. (Lu and Tm): Lu (14 4f), Tm (13 4f) } \implies \text{No Match}
💡 Teacher's Secret Hint

Remember to account for the 5d1 electron in Gd and Lu, which affects the 4f count in some cases, but here it's about the number of electrons in the 4f orbital itself.

Step 3: Identify the correct option○ Expand

Based on the evaluation, pairs A and C have the same number of electrons in their 4f orbital. Therefore, the correct answer is 'A and C Only', which corresponds to option 4.

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