How many valence electrons does the Cl⁻ anion have?
A) 17
B) 7
C) 8
D) 6
E) 35
IMAT 2026 · Question 36
Q36 - Atomic structure
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The correct answer is C) 8.
Chlorine has Z = 17, so the neutral atom is 2, 8, 7 (outer shell 3s² 3p⁵): 7 valence electrons. Cl⁻ has gained one electron, giving 2, 8, 8, the configuration of argon, with 8 valence electrons.
A) Incorrect: 17 is the atomic number (protons). Cl⁻ has 18 electrons in total, of which only 8 are valence electrons.
B) Incorrect: 7 is the number for the neutral Cl atom, before it gains an electron.
C) Correct: Cl⁻ has 8 valence electrons (3s² 3p⁶).
D) Incorrect: 6 does not match chlorine or its ion.
E) Incorrect: 35 is the mass number of the most common chlorine isotope.
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