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The mass number of an atom is defined as the sum of which two subatomic particles?
AProtons and electrons
BElectrons and neutrons
CProtons and neutrons
DNeutrons and positrons
Answer & Solution
Correct answer: C. Protons and neutrons
1. Electrons have negligible mass, so they are not counted in mass number.
2. Protons and neutrons each have a mass of about 1 amu.
3. Mass number is therefore the sum of the number of protons and neutrons.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 7 Atoms and Molecules "Atom and Atomic Mass", p.99_
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