Electromagnetic Radiation and Atomic Spectra

• Electromagnetic radiation can be described in terms of waves and characterised in terms of wavelength and/or frequency.

  The relationship between these quantities is given by c = fλ.
• The different types of radiation arranged in order of wavelength is known as the electromagnetic spectrum.
• Wavelengths of visible light are normally expressed in nanometres (nm).
• Electromagnetic radiation can be described as a wave (has a wavelength and frequency), and as a particle, and is said to have a dual nature.
• When electromagnetic radiation is absorbed or emitted by matter it behaves like a stream of particles. These particles are known as photons.
• A photon carries quantised energy proportional to the frequency of radiation.
• When a photon is absorbed or emitted, energy is gained or lost by electrons within the substance.
• The photons in high frequency radiation can transfer greater amounts of energy than photons in low frequency radiation.
• The energy associated with a single photon is given by:

  E = hf or E = hc/λ
• The energy associated with one mole of photons is given by:

  E = Lhf or E = Lhc/λ
• Energy is often in units of kJ mol-1
• When energy is transferred to atoms, electrons within the atoms may be promoted to higher energy levels.
• An atom emits a photon of light energy when an excited electron moves from a higher energy level to a lower energy level.
• The light energy emitted by an atom produces a spectrum that is made up of a series of lines at discrete (quantised) energy levels. This provides direct evidence for the existence of these energy levels.
• Each element in a sample produces characteristic absorption and emission spectra. These spectra can be used to identify and quantify the element.
• In absorption spectroscopy, electromagnetic radiation is directed at an atomised sample. Radiation is absorbed as electrons are promoted to higher energy levels.
• An absorption spectrum is produced by measuring how the intensity of absorbed light varies with wavelength.
• In emission spectroscopy, high temperatures are used to excite the electrons within atoms.

  As the electrons drop to lower energy levels, photons are emitted.

  An emission spectrum of a sample is produced by measuring the intensity of light emitted at different wavelengths.
• In atomic spectroscopy, the concentration of an element within a sample is related to the intensity of light emitted or absorbed.