2.2.2 Collisions of Electrons with Atoms

Electron Energy Levels in Atoms

Electrons within an atom occupy discrete energy levels. These are fixed energy states that electrons cannot exist between. When an electron gains energy (often through collision with a free electron), it can move to a higher energy level—a process known as excitation. Alternatively, if an electron gains enough energy to completely leave the atom, the atom becomes ionised. The minimum energy required to remove an electron from the atom entirely is known as the ionisation energy.

Excitation and Photon Emission

When an electron is excited to a higher energy level, it will eventually return to its ground state (original energy level). As it returns, it releases the energy it absorbed in the form of a photon. The energy of this photon corresponds to the difference between the two energy levels.

Energy Units: The Electron Volt ($eV$)

In atomic physics, energy differences are often very small, so a unit called the electron volt (eV) is used instead of joules ($J$).

• Definition: 1 electron volt ($1\ eV$) is the energy gained by an electron when it moves through a potential difference of $1$ volt.
• Conversion:
  • $1\ eV$ = $1.6 \times 10^{-19} J$
  • To convert joules to electron volts, divide the energy in joules by $1.6 \times 10^{-19}$.
  • To convert electron volts to joules, multiply the energy in $eV$ by $1.6 \times 10^{-19}$.