Question

Which one of the following is not isoelectronic with other isoelectronic species?

• A. $Ca^{2+}$
• B. $O^{2-}$
• C. $Mg^{2+}$
• D. $Na^{+}$
• E. $F^{-}$

Hint:

Always check the atomic number ($Z$) and adjust for the charge to find total electrons.

Answer 1

The Correct Option is A

Step 1: Concept
Isoelectronic species have the same number of electrons.

Step 2: Meaning
Calculate electrons for each:
$O^{2-} (8+2=10)$, $Mg^{2+} (12-2=10)$, $Na^{+} (11-1=10)$, $F^{-} (9+1=10)$.

Step 3: Analysis
$Ca^{2+} (20-2=18)$. It has 18 electrons, while others have 10.

Step 4: Conclusion
Hence, correct answer is $Ca^{2+}$.
Final Answer: (A)