WBCS Preliminary Examination 2024 — General Studies · Question 83 of 200

Answer: (A) Proton + Neutron

Nearly all the mass of an atom lies in its nucleus, which contains protons and neutrons; the mass number is $A = Z + N$ (protons plus neutrons). An electron is about $\frac{1}{1836}$ as heavy as a proton, so its contribution is ignored, which rules out options B, C and D. Related facts: the atomic number $Z$ (number of protons) fixes the identity of an element; isotopes have the same $Z$ but different $A$; isobars have the same $A$ but different $Z$; isotones have the same number of neutrons. Discoveries: electron, J.J. Thomson (1897); proton, Rutherford; neutron, James Chadwick (1932); nucleus, Rutherford's gold-foil experiment (1911).