* Photoelectric effect: When a photon of light strikes an atom, it can transfer its energy to an electron, causing the electron to be ejected from the atom. The minimum amount of energy required to eject an electron is called the work function.
* Thermionic emission: When an atom is heated, its electrons gain kinetic energy. If the temperature is high enough, the electrons can gain enough energy to overcome the work function and be emitted from the atom.
* Field emission: When a strong electric field is applied to an atom, it can induce electron tunneling, which is the process by which electrons can pass through a potential barrier even if they don't have enough energy to overcome it.
* Ionization: When an atom is ionized, it loses one or more electrons. This can happen when the atom is struck by a high-energy particle, such as a proton or neutron, or when the atom is exposed to a strong electric field.
Once an electron has been ejected from an atom, it is free to move around in space. These free electrons are what carry electricity and power many of our electronic devices.
