Collisions of electrons with atoms: How does ionization occur?

When a free electron travels through a gas, it may collide with neutral gas atoms.

What happens next depends entirely on how much energy the electron has.

Generally, there are three possible outcomes:

1. Elastic Collision – Just a Gentle Bump

If the electron’s energy is low, it simply bounces off the atom, losing very little energy.

The atom remains almost unaffected — like two marbles bumping into each other and rolling away — no light, no ionization, nothing dramatic.

2. Excitation – The Atom Gets “Excited”

When the electron carries more energy, it can transfer part of that energy to the atom, pushing it into an excited state.

An excited atom is unstable; when it returns to its normal (ground) state, it emits a photon — light.

This process forms the physical basis of gas-discharge illumination.

3. Ionization – The Atom Is “Broken Apart”

If the electron’s energy is even higher, it can knock one of the atom’s own electrons free, leaving behind a positive ion*and producing an additional free electron.

This is called ionization, and it’s the key process behind electrical discharges in gases.