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

1 Answer
Stefan V.
Apr 11, 2015

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.

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