alleryStart learning →Mass number of an atom is the sum of its:
AProtons and neutrons
BProtons and electrons
CElectrons and neutrons
DElectrons only
Answer & Solution
Correct answer: A. Protons and neutrons
1. Mass number A counts heavy particles in the nucleus.
2. Electrons are extremely light and ignored in A.
3. So A = protons + neutrons.
4. Often written as 'A = p + n'.
_Source: Selina Concise Chemistry ICSE Class 9, Ch 4 'Atomic Structure'_
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