With the emission of a β-particle from nucleus B, number of protons will increase by one, i.e., number of protons = 82 + 1 = 83 and number of neutrons will be 126 – 1 = 125, i.e., the nucleus C will have 83 protons and 125 neutrons.
Mass number of nucleus A = Number of protons + number of neutrons
= 84 + 128 = 212.
