Question

Which one of the following groupings represents a collection of isoelectronic species? (At. nos.: $\mathrm{Cs}=55,\mathrm{Br}=35$)

• A. ${\mathrm{Na}}^{+},{\mathrm{Ca}}^{2+},{\mathrm{Mg}}^{2+}$
• B. $\mathrm{Be},{\mathrm{Al}}^{3+},{\mathrm{Cl}}^{–}$
• C. ${\mathrm{N}}^{3-},{\mathrm{F}}^{–},{\mathrm{Na}}^{+}$
• D. ${\mathrm{Ca}}^{2+},{\mathrm{Cs}}^{+},\mathrm{Br}$

Answer 1

The correct option is C

${\mathrm{N}}^{3-},{\mathrm{F}}^{–},{\mathrm{Na}}^{+}$

Explanation for the correct option:

(c ) ${\mathrm{N}}^{3-},{\mathrm{F}}^{–},{\mathrm{Na}}^{+}$

• Isoelectronic species are those which contain the same number of electrons.
• $$\begin{gathered} {\mathrm{N}}^{3-}=(7+3)=10 \\ {\mathrm{F}}^{-}=(9+1)=10 \\ {\mathrm{Na}}^{+}=(11-1)=10 \end{gathered}$$
• Hence this option is correct

Explanation for the incorrect options:

(a) ${\mathrm{Na}}^{+},{\mathrm{Ca}}^{2+},{\mathrm{Mg}}^{2+}$

• Isoelectronic species are those which contain the same number of electrons.
• $$\begin{gathered} {\mathrm{Na}}^{+}=(11-1)=10 \\ {\mathrm{Ca}}^{2+}=(20-2)=18 \\ {\mathrm{Mg}}^{2+}=(12-2)=10 \end{gathered}$$
• Hence this option is not correct

(b) $\mathrm{Be},{\mathrm{Al}}^{3+},{\mathrm{Cl}}^{–}$

• Isoelectronic species are those which contain the same number of electrons.
• $$\begin{gathered} \mathrm{Be}=4 \\ {\mathrm{Al}}^{3+}=(13-3)=10 \\ {\mathrm{Cl}}^{-}=(17+1)=18 \end{gathered}$$
• Hence this option is not correct

(d) ${\mathrm{Ca}}^{2+},{\mathrm{Cs}}^{+},\mathrm{Br}$

• Isoelectronic species are those which contain the same number of electrons.
• $$\begin{gathered} {\mathrm{Ca}}^{2+}=(20-2)=18 \\ {\mathrm{Cs}}^{+}=(55-1)=54 \\ \mathrm{Br}=35 \end{gathered}$$
• Hence this option is not correct

Hence the correct option is (c ) ${\mathrm{N}}^{3-},{\mathrm{F}}^{–},{\mathrm{Na}}^{+}$