The number of 4f electrons in the ground statement electronic configuration of Gd²⁺ is ___________. [Atomic number of Gd = 64]

Solution

Chemistry ma’am · 3 steps

Gd keeps a half-filled 4f shell, and the 2+ ion loses only its two 6s electrons.

1. Neutral Gd. Gd has 64 electrons, so after the xenon core (Z = 54) there are 10 left. Gd is written as [Xe] 4f⁷ 5d¹ 6s², not 4f⁸ 6s², because a half-filled 4f⁷ set is extra stable.
2. Remove two electrons. Ions lose electrons from the outermost shell first. Here that is n = 6, so both 6s electrons go first.

   Gd²⁺: [Xe] 4f⁷ 5d¹

3. So. The 4f subshell is untouched, and Gd²⁺ has 7 electrons in it.

The common slip: 8. Writing Gd as [Xe] 4f⁸ 6s² and then removing the two 6s electrons gives 8, but Gd keeps 4f⁷ 5d¹ for half-filled stability.

The idea to keep. For lanthanides, watch for half-filled 4f⁷ stability (Gd, Eu) when you write the atom. Then ions always lose the highest-n electrons first, 6s before 5d or 4f.

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