Arrange the following elements in the increasing order of number of unpaired electrons in it.
(A) Sc
(B) Cr
(C) V
(D) Ti
(E) Mn
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Write each configuration, and remember chromium steals one 4s electron to half-fill its 3d.

1. Where to look. In these first-row transition metals, the unpaired electrons sit in the 3d (and sometimes 4s) orbitals. The five 3d orbitals each take one electron before any pairing starts (Hund's rule).

   Element	Configuration	Unpaired electrons
   (A) Sc	[Ar] 3d¹ 4s²	1
   (D) Ti	[Ar] 3d² 4s²	2
   (C) V	[Ar] 3d³ 4s²	3
   (B) Cr	[Ar] 3d⁵ 4s¹	5 + 1 = 6
   (E) Mn	[Ar] 3d⁵ 4s²	5 (4s is paired)

2. The chromium exception. A half-filled 3d⁵ set is extra stable, so Cr takes 3d⁵ 4s¹ instead of 3d⁴ 4s². That leaves 5 unpaired d electrons plus 1 unpaired s electron, one more than Mn.
3. So. The order is Sc (1) < Ti (2) < V (3) < Mn (5) < Cr (6), that is (A) < (D) < (C) < (E) < (B), option (b).

The idea to keep. Count unpaired electrons from the real configuration, not the Aufbau guess. Cr (3d⁵ 4s¹) and Cu (3d¹⁰ 4s¹) are the two exceptions in the 3d series, and Cr with 6 has the most.

Answer from the NTA final answer key. Solution by PrepWiser.