A radioactive atom can decay by emitting a beta particle which is a fast moving electron. A natural example of beta emission is the decay of carbon-14 into nitrogen-14. The equation for the decay is:Notice that the equation balances for both the atomic number and the atomic mass number. The atomic mass number does not change because a beta particle has a much smaller mass than the atom. The atomic number goes up because aneutron has turned into an extra proton.