Radioactivity and mass number: for which emissions does the mass number of a nucleus remain unchanged?

Introduction:
Different radioactive emissions alter the composition of a nucleus in different ways. Knowing which emissions change the mass number is vital in balancing nuclear equations and predicting daughter nuclides.

Given Data / Assumptions:

• Mass number A counts total nucleons (protons + neutrons).
• We consider α, β, and γ emissions in their common forms.

Concept / Approach:
Alpha emission ejects a helium nucleus (2 protons + 2 neutrons), decreasing mass number by 4. Beta emission converts a neutron to a proton (β−) or vice versa (β+), changing Z but not the total number of nucleons; thus, A remains unchanged. Gamma emission is the release of a photon from an excited nucleus and does not change Z or A.

Step-by-Step Solution:
α: A decreases by 4 → mass number changes.β: neutron↔proton transformation; A unchanged.γ: energy-only de-excitation; A unchanged.Therefore, emissions that do not change A: β and γ.

Verification / Alternative check:
Writing balanced nuclear equations for typical decays confirms that only α alters mass number by removing nucleons.

Why Other Options Are Wrong:

• α and β or γ and α: include α, which changes A.
• All three: incorrect because α changes A.
• Only γ: incomplete; β also leaves A unchanged.

Common Pitfalls:
Assuming β emission removes or adds particles to the nucleus. It changes identity (Z) but not the nucleon count.

Final Answer:
β and γ emissions