Question

Which of the following changes occur due to the emission of $\alpha$-particle from an atom of an element?

• A. Atomic number and mass number increase by 2 units and 4 units, respectively.
• B. Atomic number and mass number decrease by 2 units and 4 units, respectively.
• C. Only atomic number increases by one unit.
• D. Only atomic number decreases by one unit.

Answer 1

The correct option is B

Atomic number and mass number decrease by 2 units and 4 units, respectively.

The explanation for the correct option: (b)

1. An alpha particle is a particle identical to a Helium-4 (${\mathrm{He}}_2^4$) atom and has 2 protons and 2 neutrons.
2. Thus, when an atom releases an alpha particle, its mass number (M) decreases by 4 and its atomic number (Z) decreases by 2.

$$\begin{gathered} {}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}\to {}_{\mathrm{Z}-2}{}^{\mathrm{A}-4}\mathrm{Y}+{}_2{}^4\mathrm{He} \\ \mathrm{Parent}\mathrm{Alpha} \\ \mathrm{atom}\mathrm{particle} \end{gathered}$$

3. Hence, after the emission of $\alpha$-particle from an atom of an element, the atomic number decrease by 2 units, and the mass number decrease by 4 units.

The explanation for the incorrect option:

a) As helium is removed with 2 protons and 2 neutrons, thus the atomic of number and mass of the number of resulting nuclei will decrease, not increase.

c) Atomic number equal to the number of protons. As the number of protons is decreasing, the atomic number will also decrease. Similarly, mass number = number of protons + number of neutrons, as after the emission of $\alpha$-particle, the number of protons, and neutrons decreases, leading to a decrease in mass number also.

d) Both atomic number and the mass number decrease.

Conclusion: The correct option is (b) atomic number and mass number decrease by 2 units and 4 units, respectively.