Question

Which of the following are isoelectronic species i.e., those having the same number of electrons?
\(Na^+, K^+, Mg^{2+}, Ca^{2+}, S^{2–}, Ar\)

Answer 1

Isoelectronic species are atoms or ions having the same number of electrons.

Step 1: Number of electrons in each

• Na (Z = 11): Na⁺ has \(11 - 1 = 10\) electrons.
• Mg (Z = 12): Mg²⁺ has \(12 - 2 = 10\) electrons.
• K (Z = 19): K⁺ has \(19 - 1 = 18\) electrons.
• Ca (Z = 20): Ca²⁺ has \(20 - 2 = 18\) electrons.
• S (Z = 16): S²⁻ has \(16 + 2 = 18\) electrons.
• Ar (Z = 18): Ar has \(18\) electrons.

Step 2: Grouping isoelectronic species

• Na⁺ and Mg²⁺ each have 10 electrons → they are isoelectronic with each other.
• K⁺, Ca²⁺, S²⁻ and Ar each have 18 electrons → they form another isoelectronic set.

Answer

The isoelectronic species are:

• Na⁺ and Mg²⁺ (10 electrons each)
• K⁺, Ca²⁺, S²⁻ and Ar (18 electrons each)