Energy levels and spectra · Year 12
Electrons in atoms can only have certain energies, called energy levels. When an electron drops to a lower level it emits a photon whose energy exactly matches the gap; when it absorbs a photon of exactly the right energy it jumps up. Each element has its own pattern of lines – a fingerprint that lets astronomers find out what distant stars are made of.
Key facts
- Photon energy = difference between levels: hf = E_upper − E_lower.
- Emission spectrum: bright lines on a dark background (hot gas). Absorption spectrum: dark lines on a continuous spectrum (cool gas in front of a hot source).
- A hot dense solid gives a continuous spectrum.
- Energy levels are negative; 0 eV means the electron has just escaped (ionisation).
- Hydrogen: E_n = −13.6 ÷ n² eV. From level n there are n(n − 1) ÷ 2 possible lines.
- Use λ = hc ÷ ΔE (ΔE in joules) to find the wavelength of a line.