When a radioactive isotope releases an alpha particle, the mass number of the atom is

a. decreased by two.

b. decreased by four.

c. increased by two.

d. increased by four.

Correct Answer: Option B i. e. decreased by four

Reason:

Alpha particle are Helium nucleus (4 2 He). Hence, when an alpha particle is emitted from the radioactive particle, for mass balance, it's atomic number decreases by 2, while atomic mass number decreases by 4. For instance, consider following radioactive disintegration process:

238 92U → 234 90Th + 4 2He

From above example, it can be seen that, upon emission of alpha particle, mass number of 'U' decreased by 4.

I believe it's B. Decreased by four. The mass number of the atom will have an amount of 4 less than the original radioactive isotope mass number, prior to the release of the alpha particle.