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

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 will show that krypton, ##Kr##, is located in period 4, group 18, and has an 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.