Ionisation:

If the amount of external energy is large enough the valence electron can gain sufficient kinetic energy (energy of movement) to be removed completely from its atomic orbit and may not be replaced by another accelerated electron. This process is known as ionisation, since an atom which now contains one more proton than can be neutralised by the remaining electrons is a positive ion. Gas-filled devices such as Neon tubes make use of this process. In a solid where atoms are close together, simple ionisation does not occur as with individual items.