Electron Energy Levels

An atom consists of a nucleus and electrons which move in orbits around the nucleus. These orbits or shells are referred to as energy levels.
Electrons can absorb energy and jump from a lower energy level to a higher energy level within an atom. This is referred to as excitation. The energy absorbed is specific and equal to the difference in energy between the two levels.
An electron at a higher energy level can also lose energy and fall back to a lower energy level. In this case, the energy is emitted, usually in the form of photons. This process is known as de-excitation.
The energy of the emitted photon corresponds to the difference in energy levels within the atom. This is the principle behind how elements emit light of specific wavelengths, forming their unique emission spectra.
An emission spectrum is a spectrum of the electromagnetic radiation emitted by a source. Each element has a unique emission spectrum, which is like its fingerprint.
When electrons transit between energy levels, they emit or absorb radiation at specific wavelengths, forming absorption spectra or emission spectra.
The energy levels within an atom are quantised - they exist at specific and fixed energies. Electrons can’t occupy energy levels between these fixed levels.

Radioactive decay is the process in which an unstable atomic nucleus loses energy by radiation. A material that spontaneously emits such radiation — which includes alpha particles, beta particles, gamma rays and conversion electrons — is considered radioactive.
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (consists of 2 protons and 2 neutrons) and transforms into an atom with a mass number 4 less and atomic number 2 less.
Beta decay is a type of radioactive decay where a beta particle (an electron or a positron) is emitted. In beta minus decay, a neutron is converted into a proton and an electron which is emitted. In beta plus decay, a proton is converted into a neutron and a positron which is emitted.
Gamma decay is a type of radioactive decay where a gamma ray (a packet of electromagnetic energy) is emitted. No change occurs in the atomic mass or atomic number, but the atom loses energy.