JEE Main 2026 — Apr 4, Shift 1 — Chemistry solution

JEE Main2026Apr 4, Shift 1Chemistry

Which of the following is correct set of 4 quantum number of $1 9^{th}$ electron in Chromium (Atomic number = $24$ ) in accordance with Aufbau principle?

A
$n = 3, ℓ = 2, m = + 2, s = + \frac{1}{2}$
B
$n = 3, ℓ = 2, m = - 2, s = + \frac{1}{2}$
C
$n = 4, ℓ = 1, m = 0, s = + \frac{1}{2}$
D
$n = 4, ℓ = 0, m = 0, s = + \frac{1}{2}$

Solution

Chromium configuration (with the half-filled $d$ stability): $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{1} 3 d^{5}$ .

Counting in Aufbau order: $1 s$ ( $2$ ), $2 s$ ( $2$ ), $2 p$ ( $6$ ), $3 s$ ( $2$ ), $3 p$ ( $6$ ) — that totals $18$ . The $1 9^{th}$ electron enters the $4 s$ subshell.

For $4 s$ : $n = 4, ℓ = 0, m = 0, s = + \frac{1}{2}$ .

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