The mass NUMBER of an atom equals:
ANeutrons only
BProtons only
CProtons + electrons
DProtons + neutrons
Answer & Solution
Correct answer: D. Protons + neutrons
Mass number (A) = number of protons + number of neutrons (the particles in the nucleus). Electrons have negligible mass and don't contribute meaningfully. So for carbon-12: A = 12 = 6 protons + 6 neutrons.
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