Chemistry, Third edition

20 · LIGHT AND SPECTROSCOPY

Energy transitions

The quantum theory restricts the energy of a molecule or atom to certain definite

energy levels (Fig. 20.3). Movements between levels are called transitions.

Movement up the ‘ladder’ of energy levels (an upward transition) is only

possible if the molecule or atom absorbsa photon whose energy exactlyequals the

energy gap (E) involved in the jump. (This requirement is called the Bohr condition.)

This process (termed absorption) is shown in Fig. 20.3. During absorption, the pho-

ton vanishes. The relationship between the energy jump and the frequency of the

absorbed light is

E=h

Emission, also shown in Fig. 20.3, is the loss of energy by the emission of photons.

The equation E=halso applies to emission, with Ebeing the energy lost in the

emission and being the frequency of the emitted light.

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Fig. 20.3The energy levels of an

isolated atom or molecule. Three

examples of transitions involving photon

absorption (dashed lines) and photon

emission (dotted lines) are shown.

Example 20.1

Violet light has a wavelength of about 400 nm. What is the

energy of one mole of photons of violet light?

Answer

If we substitute hNAforhin the equation E=h, then Ewill have the units of kJ

per mole of photons.

h NAc 3.99 10 –13kJ s mol–13.00 108 ms–1

E= ——— = ———————————————————— = 299 kJ mol–1

 400  10 –9m

Therefore, the energy of the photons is about 300 kJ mol–1

Comment

The energy of chemical bonds are typically 200–400kJmol–1. So we should not be

surprised if the taking in of violet (and the even more energetic UV light) by

molecules breaks chemical bonds and so brings about chemicalchange. Now try

Exercise 20B.

Photon energies

Calculate the energy of a

mole of photons of (i)red

light of wavelength 700 nm

and(ii)yellow light of

wavelength 560 nm.

Exercise 20B

Bohr condition

Two energy levels in an atom are 100 kJ mol–1apart. What frequency of light needs to be

absorbed in order for 1 mol of atoms to move from the lower to the upper level?

Exercise 20C