Bohr energy levels
Electrons in hydrogen have fixed energies, E = -13.6 eV / n squared. Light is emitted when an electron drops a level.
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Analogy
Stairs rather than a ramp: no standing between steps.
Example
E = 13.6(1/4 - 1/9)= 1.89 eVlambda = 1240 / 1.89 = 656 nm
In practice
A drop from n = 3 to n = 2 emits a red photon of 1.89 eV (656 nm).
Practice
Flashcards
- What energy does hydrogen have in level n?E = -13.6 eV / n^2.
Questions people ask
Why do elements give off specific colours?
Electrons can only have certain energy levels in an atom. When one drops from a higher to a lower level, it emits a photon whose energy equals the gap. Each element has different gaps, so its light has a set of sharp lines, like a fingerprint.
What happens at the ionisation energy?
Hydrogen's ground state is -13.6 eV, so an electron needs 13.6 eV to be freed completely. Any photon with at least this energy can ionise a hydrogen atom, leaving a free proton and electron. Extra energy goes into the electron's kinetic energy.