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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+)

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.