Which of the following ions is not isoelectronic with the noble gas krypton?

Question

Quizplus · Accepted Answer

To be isoelectronic with krypton, an ion needs to have the same number of electrons as krypton. Krypton has 36 electrons. 
Ga3+ has lost 3 electrons, so it only has 33 electrons. 
Se2- has gained 2 electrons, so it has 38 electrons. 
Br- has gained 1 electron, so it has 37 electrons. 
Sr2+ has lost 2 electrons, so it only has 34 electrons. 
Zr4+ has lost 4 electrons, so it only has 32 electrons. 
Therefore, the only ion that is not isoelectronic with krypton is Ga3+.

Which of the Following Ions Is Not Isoelectronic with the Noble