An electron moves from a higher energy level to a lower one. What happens?

A photon is emitted. The photon carries away the energy difference.

What does an absorption line spectrum look like?

dark lines on a continuous spectrum. A cooler gas has absorbed its own particular wavelengths.

An atom has three energy levels. How many different photon energies can it emit?

3. Top to middle, middle to bottom, and top to bottom.

Why does each element have its own line spectrum?

Each element has its own set of energy levels. The lines are the differences between the levels.

Energy levels and line spectra · Cambridge A Level Physics

In plain words

Inside an atom an electron can't have just any energy: only certain exact amounts are allowed, like the rungs of a ladder. When it drops from one rung to a lower one, it gives out a photon carrying exactly the difference. That is why every element glows with its own exact colours, and why a line spectrum works like a fingerprint.

4 things to know

Electrons in isolated atoms have discrete energy levels. A drop from level E₁ to level E₂ emits a photon with hf = E₁ − E₂. Absorbing a photon of exactly that energy lifts the electron back up.
An emission line spectrum is a set of bright lines at particular wavelengths, given out by a hot gas.
An absorption line spectrum is a set of dark lines on a continuous spectrum, where a cooler gas has absorbed those same wavelengths from white light passing through it.
Energy levels are given as negative values, with zero for an electron that has just escaped from the atom.

Worked example

An electron in a hydrogen atom falls from the −1.5 eV level to the −3.4 eV level. Find the wavelength of the photon emitted.

Energy of the photon = (−1.5) − (−3.4) = 1.9 eV = 1.9 × 1.60 × 10⁻¹⁹ = 3.04 × 10⁻¹⁹ J.
λ = hc ÷ E = 6.63 × 10⁻³⁴ × 3.00 × 10⁸ ÷ 3.04 × 10⁻¹⁹.
= 6.5 × 10⁻⁷ m (650 nm, which is red light).

Tips and tricks

A bigger drop gives a photon of higher frequency and shorter wavelength.
Change electronvolts into joules before you use E = hf.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Energy levels and line spectra: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

An electron moves from a higher energy level to a lower one. What happens?
- A photon is absorbed.
- A photon is emitted. (the answer)
- The atom is ionised.
- Nothing.
The photon carries away the energy difference.
What does an absorption line spectrum look like?
- bright lines on a dark background
- dark lines on a continuous spectrum (the answer)
- a continuous band of colour
- a single bright line
A cooler gas has absorbed its own particular wavelengths.
An atom has three energy levels. How many different photon energies can it emit?
- 1
- 2
- 3 (the answer)
- 6
Top to middle, middle to bottom, and top to bottom.
Why does each element have its own line spectrum?
- Each element has its own set of energy levels. (the answer)
- Each element is a different colour.
- Each element is at a different temperature.
- Each element has a different number of photons.
The lines are the differences between the levels.
An electron falls from a level at −2.0 eV to one at −5.0 eV. What is the energy of the photon emitted?
- 2.0 eV
- 3.0 eV (the answer)
- 5.0 eV
- 7.0 eV
(−2.0) − (−5.0).

5 questions

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Worksheet

3 questions, 8 marks. Write your answers on paper, then check them.

Energy levels and line spectra

Cambridge International AS & A Level Physics 9702 · 8 marks · papermunch.org

Name ______________________________ Date ______________

An electron falls from the −0.85 eV level to the −3.4 eV level. Find the frequency of the photon emitted.[3]
Show answerHide answer
6.2 × 10¹⁴ Hz. The energy is 2.55 eV = 4.08 × 10⁻¹⁹ J.
I got this one
Explain why the emission spectrum of a gas is made up of separate lines.[2]
Show answerHide answer
Electrons in the atoms can only have certain discrete energies. Each line comes from a transition between two of those levels, which gives a photon of one particular energy and so one particular wavelength.
I got this one
Explain how the dark lines in an absorption spectrum are formed.[3]
Show answerHide answer
White light passes through a cooler gas. Photons whose energy exactly matches the difference between two energy levels are absorbed, lifting electrons to higher levels. The energy is given out again in all directions, so those wavelengths are missing from the light that carries straight on.
I got this one

Answers: Energy levels and line spectra

1. 6.2 × 10¹⁴ Hz. The energy is 2.55 eV = 4.08 × 10⁻¹⁹ J.
2. Electrons in the atoms can only have certain discrete energies. Each line comes from a transition between two of those levels, which gives a photon of one particular energy and so one particular wavelength.
3. White light passes through a cooler gas. Photons whose energy exactly matches the difference between two energy levels are absorbed, lifting electrons to higher levels. The energy is given out again in all directions, so those wavelengths are missing from the light that carries straight on.