Beta decay of an atom with atomic number 53 and mass number 131 yields a residual atom with which Z and A?

• A. 54, 131
• B. 53, 131
• C. 53, 129
• D. 55, 131

Answer: (A)

Beta decay involves a neutron converting into a proton with emission of an electron and an antineutrino. This increases the atomic number by one while leaving the mass number unchanged, since the total number of nucleons stays the same. Starting with atomic number 53 and mass number 131, the new nucleus has atomic number 54 and mass number 131. So the residual atom has Z = 54 and A = 131 (the element would be xenon-131).