Question

Which among the following pairs are not having same number of total electrons?

• A. $\mathrm{Na}^+$ and $\mathrm{Al}^{3+}$
• B. $\mathrm{O}^{2-}$ and $\mathrm{F}^{-}$
• C. $\mathrm{Mg}^{2+}$ and $\mathrm{Ar}$
• D. $\mathrm{P}^{3-}$ and $\mathrm{Ar}$

Hint:

Always calculate the actual number of electrons after adding or removing electrons due to ionic charge.

Answer 1

The Correct Option is C

Concept: Species having the same number of electrons are called

isoelectronic species. To find the number of electrons: \[ \text{Electrons in cation} = Z-\text{positive charge} \] \[ \text{Electrons in anion} = Z+\text{negative charge} \]

Step 1: Check option (A). \[ \mathrm{Na}^+ : 11-1=10 \] \[ \mathrm{Al}^{3+}:13-3=10 \] Same number of electrons.

Step 2: Check option (B). \[ \mathrm{O}^{2-}:8+2=10 \] \[ \mathrm{F}^{-}:9+1=10 \] Same number of electrons.

Step 3: Check option (C). \[ \mathrm{Mg}^{2+}:12-2=10 \] \[ \mathrm{Ar}:18 \] Not equal.

Step 4: Check option (D). \[ \mathrm{P}^{3-}:15+3=18 \] \[ \mathrm{Ar}:18 \] Same number of electrons. Hence only option (C) is not isoelectronic. center minipage0.45

$\mathrm{Mg}^{2+}$ and $\mathrm{Ar}$ do not have same electrons minipage center