Atoms and ions having the same number of electrons are called isoelectronic species.

(i) F–ion has 9 + 1 = 10 electrons. Thus, the species isoelectronic with it will also have

10 electrons. Some of its isoelectronic species are Na+ ion (11 – 1 = 10 electrons), Ne (10

electrons), O${}^2$–ion (8 + 2 = 10 electrons), and Al${}^{3+}$ ion (13 – 3 = 10 electrons).

(ii)Ar has 18 electrons. Thus, the species isoelectronic with it will also have 18 electrons.

Some of its isoelectronic species are S2–ion (16 + 2 = 18 electrons), Cl–ion (17 + 1 = 1

electrons), K+ ion (19 – 1 = 18 electrons), and Ca2+ ion (20 – 2 = 18 electrons).

(iii) Mg${}^{2+}$ ion has 12 – 2 = 10 electrons. Thus, the species isoelectronic with it will

also have 10 electrons. Some of its isoelectronic species are F–

ion (9 + 1 = 10 electrons),

Ne (10 electrons), O2–ion (8 + 2 = 10 electrons), and Al3+ ion (13 – 3 = 10 electrons).

(iv) Rb${}^{+}$ ion has 37 – 1 = 36 electrons. Thus, the species isoelectronic with it will

also have 36 electrons. Some of its isoelectronic species are Br–ion (35 + 1 = 36

electrons), Kr (36 electrons), and Sr2+ ion (38 – 2 = 36 electrons).