Cl⁻ is not isoelectronic with
- (a)K⁺
- (b)Mg²⁺
- (c)S²⁻
- (d)P³⁻
Correct — B, Mg²⁺. Isoelectronic species are those with the same number of electrons. A chlorine atom has 17 electrons, so the chloride ion Cl⁻, having gained one, has 18. Count the others the same way. Potassium has 19 electrons and K⁺ has 18. Sulphur has 16 and S²⁻ has 18. Phosphorus has 15 and P³⁻ has 18. All three of those match the chloride ion and share the argon configuration. Magnesium has 12 electrons, and Mg²⁺, having lost two, has 10 — the neon configuration. Ten is not eighteen, so magnesium ion is the odd one out.
- (a)K⁺ — Isoelectronic with Cl⁻. Potassium's 19 electrons minus the one it loses leaves 18, the same argon configuration.
- (c)S²⁻ — Isoelectronic with Cl⁻. Sulphur's 16 electrons plus the two it gains make 18.
- (d)P³⁻ — Isoelectronic with Cl⁻ as well. Phosphorus has 15 electrons and gains three, reaching 18.
An ion's electron count is the atomic number of the element adjusted for charge — subtract for a positive charge, add for a negative one. Species that end up with the same count are called isoelectronic, and they share an electron configuration even though their nuclei differ. The commonest isoelectronic families in school chemistry are the ten-electron neon set — N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺, Al³⁺ — and the eighteen-electron argon set, which is P³⁻, S²⁻, Cl⁻, Ar, K⁺ and Ca²⁺.
The method is mechanical and takes about ten seconds: write the atomic number, add or subtract the charge, compare. Nothing about chemical similarity is being tested, which is what makes the question fair — you do not need to know anything about magnesium beyond its position as element 12. The one place students slip is the sign. A negative charge means electrons were gained, so the count goes up; a positive charge means electrons were lost, so it goes down. Get that backwards and Mg²⁺ appears to have 14 electrons and the whole question falls apart. A useful extension to carry forward is that within an isoelectronic series the ionic radius shrinks as nuclear charge rises, so among P³⁻, S²⁻, Cl⁻ and K⁺ the phosphide ion is the largest and the potassium ion the smallest.
- Isoelectronic species have the same number of electrons.
- Cl⁻, K⁺, S²⁻, P³⁻, Ar and Ca²⁺ all have 18 electrons and the argon configuration.
- Mg²⁺ has 10 electrons, the neon configuration, sharing it with N³⁻, O²⁻, F⁻, Na⁺ and Al³⁺.
- Adding a negative charge raises the electron count; adding a positive charge lowers it.
- Across an isoelectronic series, ionic radius decreases as the nuclear charge increases.
- Reversing the sign — a negative ion has more electrons than the neutral atom, not fewer.
- Comparing chemical behaviour instead of counting electrons; isoelectronic is purely a number.
- Assuming ions with the same electron count are the same size; radius falls as nuclear charge rises.
As an odd-one-out among ions, as a which-pair-is-isoelectronic question, or as an ordering of isoelectronic ions by size.
No directly related past PYQ was found.
- practice — not a real PYQ
Which one of the following ions is isoelectronic with the neon atom?
- (a)Cl⁻
- (b)K⁺
- (c)Al³⁺
- (d)S²⁻
Answer(c) Al³⁺ — aluminium is element 13 and loses three electrons, leaving 10; the other three all have 18.
- practice — not a real PYQ
Among the isoelectronic species P³⁻, S²⁻, Cl⁻ and K⁺, which one has the smallest ionic radius?
- (a)P³⁻
- (b)S²⁻
- (c)Cl⁻
- (d)K⁺
Answer(d) K⁺ — all four hold 18 electrons, so the one with the largest nuclear charge pulls them in tightest.