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Asked in CBSE AIPMT 1999 · Q-value, decay and conservation laws
An α-particle carries away 4 nucleons, of which 2 are protons: A→ A-4 and Z→ Z-2.
Each β⁻ emission turns a neutron into a proton, raising Z by 1 and leaving A untouched.
Two β⁻ emissions therefore add 2 back to the atomic number.
Net change in Z: -2+2=0.
So the mass number falls by 4 while the atomic number remains unchanged.
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