Physics › Quantum phenomena › Collisions of electrons with atoms
Collisions of electrons with atoms
An atom cannot be nudged. Hit it with an electron and either nothing happens, or it absorbs one exact energy-level gap, or it loses an electron entirely. That all-or-nothing rule runs the fluorescent tube and defines the electronvolt.
IN THIS TOPIC
- Distinguish excitation from ionisation in collisions between electrons and atoms.
- Explain how excitation and ionisation operate inside a fluorescent tube.
- Use the electronvolt, converting between eV and joules in both directions.
WHAT YOU PROBABLY THINK
Any collision can nudge an atom just a little.
All or nothing
The electrons in an atom occupy fixed energy levels, so the atom can only accept energy in exact level-sized amounts. A free electron colliding with an atom therefore has two productive options. Excitation: the collision hands over exactly the gap to a higher level, promoting an atomic electron, and the incident electron flies on with the change. Ionisation: the collision supplies at least enough to free an atomic electron from the atom altogether.
Offer the atom less than its smallest available gap and the collision is elastic: the electron bounces off and the atom keeps none of the energy. There is no such thing as slightly warming one atom's electrons.
The fluorescent tube
AQA's named application chains both processes together. A high pd across the tube accelerates free electrons; ionisation by collision keeps the mercury vapour supplied with those free electrons. Collisions also excite mercury atoms, which promptly de-excite and emit photons, mostly ultraviolet.
Ultraviolet is no use for lighting a kitchen, so the tube's inner phosphor coating absorbs the UV photons, its own electrons climbing and then descending in smaller steps, re-emitting the energy as visible light. Every arrow in the chain is an exact energy gap changing hands.
The electronvolt
Atomic energies are absurdly small in joules, so this scale uses its own unit. One electronvolt is the energy gained by an electron accelerated through a pd of one volt: 1 eV = 1.60 × 10−19 J. The definition doubles as a mental shortcut: an electron crossing 500 V gains 500 eV, no calculation needed.
AQA expects fluent conversion both ways. Multiply by 1.60 × 10−19 to reach joules; divide to come back. The traffic is constant in this unit, because level diagrams speak eV while h and every SI formula speak joules.
THE EXAM BIT
- Definitions to the letter: excitation moves an electron to a higher energy level; ionisation removes it from the atom. Vague talk of “gaining energy” scores neither.
- The fluorescent-tube story has fixed beats: acceleration by the pd, excitation of mercury by collision, UV emission on de-excitation, absorption by the coating, visible re-emission. Missing the UV step is the classic dropped mark.
- An electron accelerated through V volts gains V electronvolts. Use the shortcut, then convert only if the question demands joules.
- Going eV → J, multiply by 1.60 × 10−19; going J → eV, divide. An energy of 1019 eV for an atomic process means the conversion ran backwards.
- If a collision offers less than the smallest gap, state the outcome: an elastic collision, with no energy absorbed by the atom.
CHECK YOURSELF
In a fluorescent tube, describe the energy story that turns the kinetic energy of a free electron into visible light. Name each process.
Show a hint
There are four hand-offs, and the ultraviolet step is the one candidates forget.
Show the answer
The tube's pd accelerates a free electron, giving it kinetic energy. A collision excites a mercury atom, transferring one exact level gap.
The atom de-excites almost at once, emitting the gap as an ultraviolet photon (hf equal to the gap).
The phosphor coating absorbs the UV, and its own electrons de-excite in smaller steps, emitting visible photons. Kinetic energy became level gaps became light, in exact parcels the whole way.
Atoms accept exact gaps, or nothing.
The tube is that rule, run as a chain.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.