Which of the following pairs are atoms and/or ions are isoelectronic? a) f-, cl- b) k+, ar c) o 2-, n 3+ d) xe, br-

Isoelectronic means that they have the same number of electrons.

b) K atom has 19 electrons (atomic number -18), K⁺ has one less electrons,  18.
Ar atom has 18 electrons (atomic number 18).

Correct answer is B.

Because a model would make the investigation easier to understand