JEE Main 2025 — Structure Of Atom Question with Solution

The ground state electron configuration of chromium (Cr, $Z=24$) is:

$$\text{Cr: }[\text{Ar}]3d^{5}4s^1$$

Let's break down the configuration:

The noble gas core $[\text{Ar}]$ represents:

$1s^2$ (with $l=0$)

$2s^2$ (with $l=0$)

$2p^6$ (with $l=1$)

$3s^2$ (with $l=0$)

$3p^6$ (with $l=1$)

Electrons with azimuthal quantum number $l=1$ are found in $p$ orbitals:

$2p^6$: 6 electrons

$3p^6$: 6 electrons

Total $p$-electrons: $6+6=12$.

Electrons with azimuthal quantum number $l=2$ are found in $d$ orbitals:

$3d^5$: 5 electrons

Thus, there are 12 electrons with $l=1$ and 5 electrons with $l=2$.

The correct option is Option B.