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SAT Chemistry

Category - Multiple Choice

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Which pair of elements and/or ions is isoelectronic?

F^-and Cl^-
O and Na
K⁺ and Ar
Mg²⁺ and S
Br and Kr

Explanation

Answer: K⁺ and Ar

Two species are isoelectronic if they have the same number of electrons. Two neutral elements cannot be isoelectronic, so the correct pair must be either an element and an ion, or two ions. The correct pair, with 18 electrons each, is K⁺ and Ar.

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