Question

Which one of the following ions is iso-electronic with krypton? (a) `Ca^(2+)`, (b) `K^+` (c) `I^-` (d) `Mg^(2+)` (e) `Sr^(2+)`

Answer 1

The answer is e) `Sr^(2+)`

For an ion to be isoelectronic with an atom (or with another ion), it must have the same number of electrons surrounding its nucleus.

A quick examination of the periodic table will show that krypton, `Kr`, is located in period 4, group 18, and has an atomic number equal to 36.

This means that a neutral krypton atom has 36 electrons surrounding its nucleus. As a result, any species that is isoelectronic ** with krypton will have 36** electrons surrounding its nucleus as well.

Start with calcium, `Ca`, which has 20 electrons surrounding its nucleus. This implies that the `Ca^(2+)` cation will have 2 electrons less, or 18 electrons.

Potassium, or `K`, has 19 electrons, which implies that `K^(+)` will have 18 electrons as well. `Ca^(2+)` and `K^(+)` are isoelectronic, but not with krypton.

Iodine, `I`, has 53 electrons, so `I^(-)` will have 54.

Magnesium, `Mg`, has 12 electrons, so `Mg^(2+)` will have 10 electrons.

Finally, strontium, `Sr`, has 38 electrons, which implies that the `Sr^(2+)` cation will have 36 electrons `->` is isoelectronic with a neutral krypton atom.