Souorce: http://ie.lbl.gov/education/glossary/AnimatedDecays/Beta-Decay.html

In beta decay a neutron in the nucleus is transformed into a proton and an electron is ejected. This causes the atomic number (Z) to increase by one while the mass number (A) remains the same.

In the example shown above an isotope of carbon decays by beta emission to become an isotope of nitrogen.

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

To distinguish this electron from the extra-nuclear electrons in the atom many authors use the Greek letter with the mass and charge numbers rather than the usual symbol for the electron. We will adopt that convention for this course.

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.

The thorium isotope produced in the previous example for the alpha decay of U-238 is also radioactive and decays by beta emission. That reaction can be represented like this: