(a) energy levels of electrons in isolated gas atoms

The idea

In an isolated gas atom, electrons can occupy only certain allowed energy levels. Each level has a definite energy, and the lowest is the ground state. An electron can move to a higher level only by absorbing an amount of energy exactly equal to the difference between the two levels, for example by absorbing a photon of that energy. It can drop to a lower level by emitting a photon with energy equal to the difference. Because the level spacings are fixed, the photons absorbed or emitted have specific energies and therefore specific frequencies and wavelengths, producing line spectra rather than a continuous spectrum.

What earns the marks

• Electrons in an isolated gas atom can occupy only discrete allowed energy levels.
• The lowest allowed level is the ground state and higher levels are excited states.
• A transition between levels involves absorption or emission of a photon with energy equal to the level difference.
• The fixed energy differences give rise to line spectra with specific frequencies and wavelengths.

Common mistakes and how to put them right

• Thinking electrons can have any energy.

  Put it right: Only discrete energy levels are allowed.

• Believing any photon can cause a transition.

  Put it right: The photon energy must match the energy difference between levels.

• Confusing excitation with ionisation.

  Put it right: Excitation moves an electron to a higher level, while ionisation removes it from the atom.

Exam tips

• Use the equation ΔE = hf to link photon energy to frequency.
• Remember that absorption produces dark lines in a spectrum and emission produces bright lines.
• Check whether the question is about an isolated atom or a solid, because energy bands apply to solids.