Question

What change in mass number occurs when a radioactive atom emits an alpha particle?

Answer 1

The alpha particle is the helium nucleus.
The mass of helium nucleus (⁴₂He) is 4u.


The mass of radioactive atom decreases by 4u. Look at the reaction below:

²²³₈₈Ra ---> ⁴₂He + ²¹⁹₈₆Rn