What ion/atoms are isoelectronic?

Question

Selenium

Magnesium

Bromine

Nitrogen

Krypton

Zinc

Barium

Iron

Bromide

Barium cation

Iron (III)

Selenide

Answer 1

To be isoelectronic, they should have the same electron configuration.

No two neutral atoms can have the same electron configuration (otherwise they'd be the same element).

So, we should look for ions and noble gases, and the only ions in the list are:

Bromide = Br^-

Barium cation = Ba²⁺

Iron (III) = Fe³⁺

Selenide = Se^2-

And the only noble gas is Krypton (Kr)

Comparing the electron configurations of these we see if you add 1 electron to Br you get Br^- and if you add 2 electrons to Se you get Se^2- and these both have the same electron configuration as each other and the same as Kr. So...

Br^-, Se^2- and Kr are isoelectronic

The Ba²⁺ cation would be isoelectronic with Xenon, but that's not on the list, and Fe³⁺ would be isoelectronic with Mn²⁺ but that's also not on the list, so I think those are the only ions and atoms that are isoelectronic.