Two main processes by which an unstable nucleus decays are alpha decay and beta decay.
In alpha decay, the unstable nucleus emits an alpha particle reducing its proton number Z as well as its neutron number N by 2. As the proton number is changed, the element itself is changed and hence the chemical symbol of the residual nucleus is different from that of the original nucleus (Parent nucleus is original nucleus and the resulting nucleus due to decay is called daughter nucleus).
Alpha decay occurs in all nuclei with mass number A>210.
Beta decay is a process in which either a neutron is converted into a proton or a proton is converted into a neutron.
When a neutron is converted into a proton, an electron and a new particle named antineutrino are created and emitted from the nucleus. The electron emitted from the nucleus is called a beta particle and is denoted by the symbol β-.
If the unstable nucleus has excess protons than needed for stability, a proton converts itself into a neutron. In the process, a positron and a neutrino are created and emitted from the nucleus.
Positron is represented by e+. The neutrino is represented by ν.
When a positron and electron collide, both the particles are destroyed and energy is made available.
The decay which gives beta rays consisting of positrons is called beta plus decay
A nucleus captures one of the atomic electrons, most likely an electron from the K shell, and a proton in the nucleus combines with the electron and converts itself into a neutron. A neutrino is created in process and emitted from the nucleus. So a combination of proton and electron results in neutron and neutrino.
When a daughter nucleus is formed due to alpha or beta decay, the nucleus may be at higher energy level compared to its ground or normal state. The electromagnetic radiation emitted in nuclear transitions from higher energy or excited state to ground state is called gamma ray.