WBJEE 2024 Physics & Chemistry Paper

To determine the number of neutrons in the atom after the emission of a β-particle and an α-particle from ‌83214‌Bi, we need to understand how these emissions affect the atomic and mass numbers.
1. Emission of a β-particle:
Aβ-particle is essentially an electron, and its emission converts a neutron into a proton. Therefore, the atomic number increases by 1 , but the mass number remains the same. For ‌83214‌Bi (bismuth), after the emission of a β-particle, the atomic number becomes 84 :
‌83214‌Bi⟶‌84214‌Po+β
2. Emission of an α-particle:
An α-particle consists of 2 protons and 2 neutrons, so its emission decreases the atomic number by 2 and the mass number by 4 . Therefore, for ‌84214‌Po (polonium), after the emission of an α-particle, the atomic number becomes 82 and the mass number becomes 210:
‌84214‌Po⟶‌82210‌Pb+α

Now, we need to find the number of neutrons in the resulting atom, ‌82210‌Pb (lead). The number of neutrons is given by the difference between the mass number and the atomic number:
Number of neutrons = Mass number - Atomic number
Number of neutrons =210−82=128
Therefore, the correct answer is:
Option B
128