3.4 Energy levels in atoms
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33 Terms
What holds electrons in an atom?
Electrostatic attraction of the nucleus
What are the allowed paths of electrons called?
Shells or orbits
Which shell has the lowest energy?
The one nearest the nucleus
How many electrons can the first shell hold?
Two
How many electrons can the second shell hold?
Eight
What is the ground state of an atom?
Its lowest energy state
What is an excited state?
When an electron moves to a higher energy shell after absorbing energy
What is the ionisation level?
The energy needed to remove an electron completely from the atom
Why is an excited electron configuration unstable?
It leaves a vacancy in a lower shell
What happens during de-excitation?
An electron falls to a lower energy level and emits a photon
What determines the energy of the emitted photon?
The difference between the two energy levels
A mercury atom at 4.9 eV de-excites. What energy photon is emitted?
4.9 eV
Can de-excitation occur in steps?
Yes, if intermediate energy levels exist
When can an electron absorb a photon?
Only if photon energy exactly matches the energy gap between levels
What happens if the photon's energy doesn't match?
It is not absorbed
Can a photon cause ionisation?
Yes, if its energy ≥ ionisation energy
What is fluorescence?
Emission of visible light after absorbing UV radiation
What happens when UV photons are absorbed?
Atoms become excited and then emit visible photons when they de-excite
Why does fluorescence stop when UV is removed?
No more excitation occurs
What causes visible light in a fluorescent tube?
UV photons excite the fluorescent coating, which then emits visible light
What gas does a fluorescent tube contain?
Low-pressure mercury vapour
What do collisions in the tube cause?
Ionisation and excitation of mercury atoms
What photons do mercury atoms emit when de-exciting?
UV, visible, and low-energy photons
What does the coating inside the tube do?
Absorbs UV photons and emits visible light
Why is a fluorescent tube more efficient than a filament lamp?
Less energy is wasted as heat
Why is a starter unit needed?
To heat electrodes for ionisation when the tube is cold
What gas is in the starter unit?
Argon
What happens when the bimetallic strip bends and opens?
The mains voltage ignites the mercury vapour
A 60 W lamp runs for 1100 h at 10% efficiency. Energy wasted?
60 kWh
Why are low-energy bulbs better than filament bulbs?
They use less power for the same light output
How much light does a 100 W filament bulb emit?
About 15 W of light, wasting the rest as heat
How much power does a low-energy bulb use for the same output?
About 15 W total, wasting ~5 W
How much more efficient is a low-energy bulb than a filament bulb?
Five times more efficient