Advanced Higher Chemistry Unit 1 Spectroscopy

Spectroscopy  Spectroscopy is used to give information regarding the structure of atoms or molecules.  When energy changes take place within atoms or molecules they give rise to the emission or absorption of radiation of definite frequencies (or wavelengths). The measurement of these frequencies gives information about the energy levels within the particle and hence, its structure.

Spectroscopy  Atomic Emission Spectroscopy (AES) or Atomic Absorption Spectroscopy (AAS) involve the transition of electrons between energy levels in atoms.  The energy differences usually correspond to the visible region (400 – 700nm) but some applications use the UV region (200 – 400nm).

Excited Atoms When atoms absorb energy, they become excited. When atoms absorb energy, they become excited. Electrons move to a higher energy. When they fall back down to ground state, a photon of light is emitted. Electrons move to a higher energy. When they fall back down to ground state, a photon of light is emitted.

Emission Spectra and Energy The difference between energy levels is fixed. The difference between energy levels is fixed. When an electron falls back to ground state, the wavelength of light emitted can be used to calculate the difference in energy. When an electron falls back to ground state, the wavelength of light emitted can be used to calculate the difference in energy.

Emission Spectra Elements have their own characteristic emission spectrum. Elements have their own characteristic emission spectrum. For example, the emission spectrum for neon has lines between 580 and 670 nm (the red region of the visible spectrum) therefore glowing neon appears red. For example, the emission spectrum for neon has lines between 580 and 670 nm (the red region of the visible spectrum) therefore glowing neon appears red.

Atomic Absorption Spectroscopy  The sample to be tested is illuminated with a range of frequencies.  If a photon of light hits an atom it may cause the atom to make a transition from lower to higher energy levels.  If the light’s frequency does not match to a higher energy state of the atom then the photon is not absorbed.

Atomic Absorption Spectroscopy  The radiation that has been passed through the sample is then passed through a diffraction grating. The frequencies absorbed appear as black lines against a spectrum of colours.  Because each element has a characteristic spectrum it can then be identified.  The frequencies of dark lines in an absorption spectrum will be identical to the coloured lines in an emission spectrum since they represent transitions in opposite directions between the same energy levels.

Atomic Emission Spectroscopy By analysing the wavelengths of the light emitted, AES can determine which elements are present in the sample. By analysing the wavelengths of the light emitted, AES can determine which elements are present in the sample.  By measuring the intensity of the light emitted, AES can also determine the quantity of the element in the sample.

Uses of emission and absorption spectroscopy  Emission spectroscopy is used in flame testing for testing group 1 and 2 metals.  Sodium vapour and mercury vapour street lights and neon signs have specific colours due to their emission spectra.  Both emission and absorption spectra are used to detect trace elements in water and food samples and in the analysis of industrial materials e.g. steels.