Which of the following statements about the Mg²⁺ ion is correct?
A) The Mg²⁺ ion has a larger radius than the Mg atom
B) The two chemical species (the ion and the neutral atom) have the same number of protons, electrons, and neutrons
C) The ion has 8 electrons in its outermost electron shell
D) The ion has two more electrons than the neutral atom
E) None of the other statements is correct
IMAT 2026 · Question 33
Q33 - Atomic structure
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The correct answer is C) The ion has 8 electrons in its outermost electron shell.
Magnesium has Z = 12, so the neutral atom has the configuration 2, 8, 2 (1s² 2s² 2p⁶ 3s²). To form Mg²⁺ it loses its two outer electrons, leaving 2, 8: the outermost shell is now the second one, with 8 electrons, like neon.
A) Incorrect: A cation is smaller than its atom. Mg²⁺ loses its entire third shell, and 12 protons now pull on only 10 electrons.
B) Incorrect: Protons and neutrons are the same, but the ion has two fewer electrons.
C) Correct: Mg²⁺ has the configuration 2, 8: 8 electrons in the outer shell.
D) Incorrect: It is the opposite: the 2+ charge means two fewer electrons.
E) Incorrect: C is correct, so this is false.
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