Particles, photons and the Bohr model

MDCAT Chemistry · Chemistry · Atomic structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An atom has protons and neutrons in a tiny nucleus, with electrons around it. The atomic number Z is the number of protons. The mass number A is protons plus neutrons. Isotopes have the same Z but different numbers of neutrons. They react in the same way because they have the same electrons.

Light carries energy in packets called photons. A photon of higher frequency, or shorter wavelength, has more energy.

In Bohr's model the electron of hydrogen moves only in fixed orbits of fixed energy. When it falls to a lower orbit it gives out one photon whose energy equals the energy gap. Each gap gives one line, so the spectrum is a line spectrum.

Rules to remember

Common mistake

Students put the wavelength into E = hc/λ in nanometres. Change it to metres first (1 nm = 10−9 m).

Worked example

What is the energy of one photon of ultraviolet light of wavelength 300 nm? (h = 6.63 × 10−34 J s, c = 3.00 × 108 m s−1)

  1. Wavelength in metres: 300 nm = 300 × 10−9 m = 3.00 × 10−7 m.
  2. Frequency f = c ÷ λ = (3.00 × 108) ÷ (3.00 × 10−7) = 1.00 × 1015 s−1.
  3. E = hf = 6.63 × 10−34 × 1.00 × 1015 = 6.63 × 10−19 J.

Answer: 6.63 × 10−19 J

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