Question 22 of 62beginner💡 UnderstandShort Answer2 marks

Which of the following are isoelectronic species i.e., those having the same number of electrons? Na⁺, K⁺, Mg²⁺, Ca²⁺, S²⁻, Ar

Correct Answer

Na⁺ and Mg²⁺ are isoelectronic species as they both have 10 electrons.

K⁺, Ca²⁺, S²⁻, and Ar are isoelectronic species as they all have 18 electrons.

Exercise: EXERCISES | Q: 2.22 | (Page 41)
For More Understanding

Explanation

To find isoelectronic species, we calculate the total number of electrons in each given ion and atom. Na has 11 electrons, so Na⁺ has 10. Mg has 12 electrons, so Mg²⁺ has 10. Thus, Na⁺ and Mg²⁺ are isoelectronic. Ar has 18 electrons. K has 19 electrons, so K⁺ has 18. Ca has 20 electrons, so Ca²⁺ has 18. S has 16 electrons, so S²⁻ has 18. Thus, K⁺, Ca²⁺, S²⁻, and Ar are isoelectronic.

Solution Steps

  1. Step 1: Calculate electrons for Na⁺ and Mg²⁺. Na has configuration 1s22s22p63s11s^2 2s^2 2p^6 3s^1 (11 electrons). Na⁺ loses 1 electron, leaving 10 electrons. Mg is the next element after Na, so it has 12 electrons. Mg²⁺ loses 2 electrons, leaving 10 electrons. Therefore, Na⁺ and Mg²⁺ are isoelectronic.

  2. Step 2: Calculate electrons for Ar, K⁺, Ca²⁺, and S²⁻. Ar has configuration 1s22s22p63s23p61s^2 2s^2 2p^6 3s^2 3p^6 (18 electrons). K is after Ar, so it has 19 electrons; K⁺ loses 1 electron, leaving 18 electrons. Ca is after K, so it has 20 electrons; Ca²⁺ loses 2 electrons, leaving 18 electrons. S is two elements before Ar, so it has 16 electrons; S²⁻ gains 2 electrons, reaching 18 electrons. Therefore, K⁺, Ca²⁺, S²⁻, and Ar are isoelectronic.

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