3.2.2.2
Electrons can excite or ionise atoms through collisions
Energy Levels & Wave-Particle Duality — AQA A-Level Physics
Key Definition
Ionisation — The removal of an electron from an atom, leaving a positive ion. The atom gains enough energy to completely free the electron.
Key Definition
Excitation — When an electron in an atom absorbs energy and moves to a higher energy level, without leaving the atom.
- When a free electron collides with an atom, it can transfer energyThe capacity to do work. Measured in joules (J). to an atomic electron.
- If the transferred energyThe capacity to do work. Measured in joules (J). exactly matches the gap between two energyThe capacity to do work. Measured in joules (J). levels, the atomic electron is excited to the higher level.
- If the transferred energy is enough to free the electron entirely, ionisation occurs.
- The minimum energy needed to ionise an atom from its ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels. is called the ionisation energyThe minimum energy required to remove an electron completely from an atom in its ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels. to infinity..
- For hydrogen, the ionisation energyThe minimum energy required to remove an electron completely from an atom in its ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels. to infinity. is 13.6 eV (the energy needed to move an electron from the ground state to $n = \infty$).
Fluorescent tubes use excitation
- Free electronsElectrons not bound to any particular atom, free to move through a conductor. Also called delocalised or conduction electrons. in the tube collide with mercury vapour atoms, exciting their electrons.
- When the excited electrons de-excite, they emit UV photons.
- The UV photons hit the phosphor coating on the tube walls, which re-emits visible light.
- This is a common exam example of excitation in practice.
Common Mistake
MEDIUM
Students often: Don't say any amount of energy can excite an electron.
Instead: The energy must exactly match the gap between two energy levels. Partial energy transfers do not cause excitation — the electron stays in its original level and the colliding electron keeps its kinetic energyThe energy an object possesses due to its motion..
Instead: The energy must exactly match the gap between two energy levels. Partial energy transfers do not cause excitation — the electron stays in its original level and the colliding electron keeps its kinetic energyThe energy an object possesses due to its motion..