Radioactive isotopes of elements have unstable arrangement of neutrons and protons in their nuclei. To become more stable, they emit alpha or beta particles. But this changes the number of protons and neutrons in the nucleus resulting in the formation of a new element. This process is called radioactive decay.
The original nucleus is called the parent nucleus. The newly formed nucleus is called the daughter nucleus. The daughter nucleus and any other particles emitted are called the decay products.
After alpha decay, a new element is formed whose atomic number is 2 less and mass number is 4 less than before.
For example, Radium-226 (atomic number 88) decays by alpha emission. The mass number drops to 222 and atomic number becomes 86. The new element formed is Radon-222.
There are 2 kinds of beta decay – beta‾ decay and beta+ decay.
In Beta‾decay, a neutron changes into a proton, an electron and an antineutrino (an uncharged, massless particle like the electron). The new element formed has an atomic number 1 more than before and the mass number remains unchanged.
In Beta+decay, a proton changes to a neutron, a positron and a neutrino. The new element formed has atomic number 1 less than before and the mass number remains unchanged.
The emission of alpha or beta particle from the nucleus leaves the protons and neutrons in an excited state. So as they rearrange to become more stable, they lose energy. This is emitted as a burst of gamma radiation.