Cl⁻ is not isoelectronic with
- A.K⁺
- B.Mg²⁺
- C.S²⁻
- D.P³⁻
Correct answer
Explanation
The correct answer is B, Mg²⁺. The chloride ion has 18 electrons, while the magnesium ion has only 10, so the two are not isoelectronic.
Isoelectronic species have the same number of electrons. Chlorine has atomic number 17, and gaining one electron gives Cl⁻ 18 electrons, the same arrangement as the noble gas argon (2, 8, 8). Magnesium has atomic number 12; losing two electrons leaves Mg²⁺ with 10 electrons, the neon arrangement (2, 8). To count electrons in an ion, take the atomic number, add one for each negative charge and subtract one for each positive charge.
A is wrong: potassium (19) loses one electron, so K⁺ has 18 electrons, like Cl⁻.
C is wrong: sulphur (16) gains two electrons, so S²⁻ has 18 electrons.
D is wrong: phosphorus (15) gains three electrons, so P³⁻ also has 18 electrons.
Exam tip: P³⁻, S²⁻, Cl⁻, Ar, K⁺ and Ca²⁺ all have 18 electrons; N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺ and Al³⁺ all have 10.
Practice Questions
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