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

Apr 11, 2015

The answer is e) $S {r}^{2 +}$

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, $K r$, 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, $C a$, which has 20 electrons surrounding its nucleus. This implies that the $C {a}^{2 +}$ 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. $C {a}^{2 +}$ and ${K}^{+}$ are isoelectronic, but not with krypton.

Iodine, $I$, has 53 electrons, so ${I}^{-}$ will have 54.

Magnesium, $M g$, has 12 electrons, so $M {g}^{2 +}$ will have 10 electrons.

Finally, strontium, $S r$, has 38 electrons, which implies that the $S {r}^{2 +}$ cation will have 36 electrons $\to$ is isoelectronic with a neutral krypton atom.