Chemistry Q67 – NEET-UG 2026

Home Chemistry d- & f-Block ElementsQuestion #375

The lanthanide ion having four unpaired electrons is (Given : Atomic numbers of Ce = 58, Nd = 60, Tb = 65 and Ho = 67)

Clue 1 of 3: Lanthanide Electronic Configuration

Lanthanides typically have a general electronic configuration of $[X e] 4 f^{n} 5 d^{0} 6 s^{2}$ , with some exceptions.

🧠 Full Solution Path

Step 1: Determine Electronic Configuration of Neutral Lanthanides✦ Active

The atomic numbers are given. We first write the electronic configuration for the neutral lanthanide elements, using Xenon ( $[X e]$ , Z=54) as the noble gas core. The general configuration is $[X e] 4 f^{n} 5 d^{0} 6 s^{2}$ , with some variations for Ce, Gd, and Lu.

Ce (Z=58):

[X e] 4 f^{1} 5 d^{1} 6 s^{2}

Nd (Z=60):

[X e] 4 f^{4} 6 s^{2}

Tb (Z=65):

[X e] 4 f^{9} 6 s^{2}

Ho (Z=67):

[X e] 4 f^{11} 6 s^{2}

Step 2: Determine Electronic Configuration and Unpaired Electrons for

L n^{3 +}

Ions○ Expand

Lanthanides typically form $+ 3$ ions by losing the two $6 s$ electrons first, followed by any $5 d$ electron, and then $4 f$ electrons if necessary to achieve the $+ 3$ charge. We then apply Hund's rule to find the number of unpaired electrons in the $4 f$ subshell (which has 7 orbitals).

^{3 +}

: Loses

6 s^{2}

and

5 d^{1} \to [X e] 4 f^{1}

. Number of unpaired electrons = 1. Nd

^{3 +}

: Loses

6 s^{2}

and one

4 f \to [X e] 4 f^{3}

. Number of unpaired electrons = 3. Tb

^{3 +}

: Loses

6 s^{2}

and one

4 f \to [X e] 4 f^{8}

. Number of unpaired electrons = 6 (7 orbitals, 7 up, 1 down). Ho

^{3 +}

: Loses

6 s^{2}

and one

4 f \to [X e] 4 f^{10}

. Number of unpaired electrons = 4 (7 orbitals, 7 up, 3 down).

💡 Teacher's Secret Hint

Remember that electrons are removed from the outermost shells first ( $6 s$ , then $5 d$ , then $4 f$ ). For $f^{n}$ configurations, the number of unpaired electrons is $n$ if $n \leq 7$ , and $14 - n$ if $n > 7$ (assuming all electrons are in $f$ orbitals).

Step 3: Identify the Ion with Four Unpaired Electrons○ Expand

Comparing the number of unpaired electrons for each ion, Ho $^{3 +}$ has four unpaired electrons.

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Chemistry Question 67 – NEET-UG 2026

Clue 1 of 3: Lanthanide Electronic Configuration

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