Particles with the same electron arrangement are said to be isoelectronic - Scottish Highers Chemistry - Question 1 - 2016

1. Na⁺: 1s² 2s² 2p⁶ (10 electrons) S²⁻: 1s² 2s² 2p⁶ (18 electrons)
2. Mg²⁺: 1s² 2s² 2p⁶ (10 electrons) Cl⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons)
3. K⁺: 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons) Br⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4p⁶ (36 electrons)
4. Ca²⁺: 1s² 2s² 2p⁶ (20 electrons) Cl⁻: 1s² 2s² 2p⁶ 3s² 3p⁶ (18 electrons).

The ions that are isoelectronic must have the same number of electrons.

• From the analysis: Na⁺ and Mg²⁺ both have 10 electrons, making them isoelectronic.
• S²⁻ and Ca²⁺ do not form a pair but Cl⁻ from both groups have the same electron configuration different from K⁺.
• K⁺, Cl⁻, and Br⁻ configurations differ and show no isoelectronic relation. Thus, the answer is based on one pair being isoelectronic.

The correct answer is D: CaCl₂, as both Ca²⁺ and Cl⁻ also reinforce the pairing of 18 electrons being consistent with the configurations wherein they collectively uphold electron arrangements similar to noble gases.