When a nucleus ejects an alpha particle, its mass number decreases by 4.

Here's why:

* Alpha particle composition: An alpha particle is essentially a helium nucleus, consisting of 2 protons and 2 neutrons.

* Mass number: The mass number of an atom is the sum of its protons and neutrons.

* Ejection process: When an alpha particle is ejected, the nucleus loses 2 protons and 2 neutrons.

Therefore, the mass number decreases by 4 (2 protons + 2 neutrons = 4).

Example:

* Uranium-238 (mass number 238) emits an alpha particle and becomes Thorium-234 (mass number 234).

* 238 - 4 = 234