MDCAT Physics · Physics · Atomic spectra. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
In the Bohr model the electron of a hydrogen atom moves round the nucleus only in certain allowed orbits, where its angular momentum is a whole number times h/2π. In an allowed orbit it does not radiate energy.
Each orbit has a fixed energy. The energies are negative because the electron is bound to the nucleus. The ground state (n = 1) is −13.6 eV. The levels get closer together as n rises and reach zero at n = ∞. The orbit radius grows as n2.
When the electron drops from a higher level n to a lower level p, it emits one photon whose energy equals the difference between the two levels. All transitions that end on the same lower level form a spectral series.
Students mix up the ends of a series. The longest wavelength comes from the smallest jump (from the next level up), and the shortest wavelength, the series limit, comes from n = ∞.
Answer: λ ≈ 4.85 × 10−7 m (about 485 nm), a visible line of the Balmer series.
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