When a radioactive atom decays, it loses energy by emitting particles or radiation. This process changes the composition of the nucleus, often transforming it into a new element.
A carbon-14 atom has six protons and eight neutrons. When it undergoes beta decay, one of its neutrons splits into a proton, an antineutrino and an electron.
The electron (also known in this case as a beta particle) and the antineutrino are ejected from the nucleus. The nucleus now has seven protons and seven neutrons – making it a nitrogen-14 atom.