Arrange the following ions in increasing order of number of 3d-electr…

The correct answer is Option (2) → (C), (A), (D), (B)

1. Determine the number of $3d$-electrons for each ion

The neutral atoms have the following atomic numbers ($Z$) and ground-state electron configurations:

• $\text{Ti} (Z=22): [\text{Ar}] 4s^2 3d^2$
• $\text{Cr} (Z=24): [\text{Ar}] 4s^1 3d^5$ (Exception)
• $\text{Mn} (Z=25): [\text{Ar}] 4s^2 3d^5$
• $\text{Cu} (Z=29): [\text{Ar}] 4s^1 3d^{10}$ (Exception)

When forming ions, electrons are always removed from the outermost shell first, which is the $4s$ orbital, before the $3d$ orbital.

Ion	Neutral Atom Configuration	Ion Charge	Final Configuration	Number of 3d-electrons
(A) $\text{Cr}^{2+}$	$4s^1 3d^5$	$+2$	$4s^0 3d^4$ (Remove $1e^-$ from $4s$, $1e^-$ from $3d$)	4
(B) $\text{Cu}^{+}$	$4s^1 3d^{10}$	$+1$	$4s^0 3d^{10}$ (Remove $1e^-$ from $4s$)	10
(C) $\text{Ti}^{3+}$	$4s^2 3d^2$	$+3$	$4s^0 3d^1$ (Remove $2e^-$ from $4s$, $1e^-$ from $3d$)	1
(D) $\text{Mn}^{+}$	$4s^2 3d^5$	$+1$	$4s^1 3d^5$ (Remove $1e^-$ from $4s$)	5

2. Arrange in Increasing Order

Now, arrange the ions based on the number of $3d$-electrons in ascending order:

Number of 3d-electrons	Ion
1	$\text{Ti}^{3+}$ (C)
4	$\text{Cr}^{2+}$ (A)
5	$\text{Mn}^{+}$ (D)
10	$\text{Cu}^{+}$ (B)

The increasing order is (C), (A), (D), (B).