Identify the element for which electronic configuration in +3 oxidation state is [Ar]3d⁵ :

Solution

Chemistry ma’am · 3 steps

Add the three lost electrons back to 3d⁵: eight electrons beyond argon means Z = 26, iron.

1. Work backwards. The ion is [Ar] 3d⁵, which is 5 electrons beyond argon. Add back the 3 lost electrons: the neutral atom has 5 + 3 = 8 electrons beyond argon, so its atomic number is 18 + 8 = 26.
2. Remove 4s first. When a 3d metal forms a cation, it loses its 4s electrons before any 3d electrons. So the first two electrons go from 4s and the third comes from 3d.

   Element	Neutral atom	+3 ion
   Ru (Z = 44)	[Kr] 4d⁷ 5s¹	[Kr] 4d⁵
   Mn (Z = 25)	[Ar] 3d⁵ 4s²	[Ar] 3d⁴
   Co (Z = 27)	[Ar] 3d⁷ 4s²	[Ar] 3d⁶
   Fe (Z = 26)	[Ar] 3d⁶ 4s²	[Ar] 3d⁵

3. So. Only iron gives exactly [Ar] 3d⁵ in the +3 state, so the answer is (d) Fe.

The idea to keep. For a 3d ion, add the charge back to the d count to find the neutral atom's electrons beyond argon. Always strip 4s before 3d.

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