An ion is a charged atom. The number of electrons in and ion is not euqal to the number or protons. An ion is formed from an uncharges atom by adding or removing electrons from the atom. Adding electrons makes the atom into a negative ion. Removing electrons makes the atom into a positive ion. This process is called ionisation.
Excitation by collision
Using gas-filled tubes with a metal grid between the filament and the anode, we can show that gas atoms can absorb energy from colliding electrons without being ionised. This process is known as excitation and happens at certain energies, which are characteristic of the atoms of the gas.…