Source: http://ie.lbl.gov/education/glossary/AnimatedDecays/AlphaDecay.html

In alpha decay two neutrons and two protons are ejected simultaneously from the nucleus in one package. This causes the atomic number (Z) to decrease by two and the mass number (A) to decrease by four.

In the example shown above an isotope of element 106, Seaborgium, decays by alpha emission to become an isotope of element 104, Rutherfordium.

The process can be represented in a balanced nuclear reaction using the isotopic symbols:

Notice that the alpha particle is written on the right side of the arrow. It is a product of the reaction. This is true for every decay process that results in a particle being ejected from the nucleus.

We can check that the reaction is balanced by comparing the sums of the lower numbers on each side and the upper numbers. There should be no net change.

This simple rule can be used to determine the product or "daughter" isotope for a decay if the type of decay is already known. For example, U-238, the most plentiful isotope of uranium, also decays by alpha particle emission. That means the mass number will decrease by 4 to become 234 and the atomic number will decrease by 2 to become 90. That is the atomic number of thorium: