Question

How does the nucleus of an atom change when it emits an alpha or beta particle?

Answer 1

First of all, an alpha particle consists of two protons and two neutrons (similar to a Helium atom). When an atom emits an alpha particle, its nucleus loses two protons and two neutrons. And its total mass decreases by 4 atomic mass units.(As mass of a proton or a neutron is 1 amu)

There are two kinds of beta emissions (or particles) positron or electron. 

When an atom emits a ${ \beta }^{ + }$ (positron) particle, actually one if its protons releases the particle. Since a positron's charge is positive, when the atom releases a positron it loses its positive charge and turns into a neutron.

When an atom emits a ${ \beta }^{ - }$ (electron) particle, one of its neutrons releases a ${ \beta }^{ - }$ (electron). And since an electron's charge is negative (-) the neutron loses its negative charge (since neutron's electrical charge is neutral) and turns into a proton.

This is simply explained ^ 

There are some other details but this should be enough :)