Energy Levels & Photon Emission

Physics: AQA A Level, topic 2.2.5

Energy Levels in Atoms

Electrons in atoms must occupy a discrete energy level.

Energy levels

Electrons do not float around randomly in atoms.
They can only be found at set energy levels.
Each energy level is given a 'quantum number'.
The ground state is the lowest energy level. The ground state has n = 1.
The higher the energy level, the larger its quantum number, n.

Transitions between energy levels

An electron can transition between energy levels by absorbing or emitting a photon of a specific frequency.
If an electron emits a photon, it will drop an energy level.
If an electron absorbs a photon, it will rise to a higher level.
In both cases, the photon must have energy exactly equal to the energy difference between the levels.

Equation

The movement of electrons between energy levels can be represented in the equation:
$\Delta E = E_2 - E_1 = hf = \frac{hc}{\lambda}$
This shows that the energy difference between levels one and two is equal to the energy of the emitted (or absorbed) photon.

Excitation and ionisation

If an electron absorbs a photon and jumps up an energy level, we say it has been excited.
If an electron absorbs a photon that is so energetic it is knocked out of the atom altogether, the electron has been ionised.

Absorption Spectra

Electrons in atoms can only move between discrete energy levels. So an atom can only absorb or emit photons with particular frequencies (or wavelengths).

White light - continuous spectrum

White light contains all the colours in the visible part of the spectrum.
When white light is passed through a prism or a diffraction grating, the colours are split up to produce a continuous spectrum.
Continuous means there aren't any gaps or black lines.

Cool gas - absorption spectra

A cool gas means a gas containing atoms in their ground state.
When white light is passed through a cool gas, electrons in the ground state of the atom absorb certain frequencies of light and become excited.
Most frequencies of the white light are not absorbed because they do not correspond to the difference between two energy levels in the atom.

Absorption spectra 2

If the light coming out of the cool gas is split by a prism (or diffraction grating), we see a continuous spectrum with black lines.
These lines are called absorption lines.
They are unique to each type of atom.

Emission Spectra

Electrons in atoms can only move between discrete energy levels. So an atom can only absorb or emit photons with particular frequencies (or wavelengths).

Excited gas

An excited gas contains atoms in excited energy states.
These excited atoms contain electrons in high energy levels.
As these electrons de-excite and fall back to lower energy levels, photons are emitted.

Photon emission

These photons carry away energy from the atoms.
The amount of energy (and so what frequency) the photons have depends on the difference in energy levels in the atom.
For every possible electron transition, there will be a unique frequency photon emitted.
We see a series of bright lines when we pass the emitted light through a prism or diffraction grating.

Emission vs absorption

Compare the emission and absorption spectra produced by this gas.
We can see that the emission lines exactly match the frequencies of the absorption lines.
This is because the lines correspond to the same energy differences in the atoms. The same energy photons are either emitted or absorbed.

Test yourself on Energy Levels & Photon Emission

What is the quantum number for the ground state of an electron?
What is the correct equation for the change in energy between energy levels?
When white light passes through a cool gas what happens?
Which of these statements about absorption spectra are true?
When an electron falls to a lower energy level what happens?

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