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vacuum, it is approximately equal to 3.00 x 108ms-¹. Frequency and wavelength are related by the formula: C = fx Example Electromagnetic radiation is found to have a wavelength of 1100nm. Calculate the frequency of this radiation (three significant figures). c = fxx f = { f = 3.00×108 1100×10-9 f = 2.73 × 10¹4 Hz X Electromagnetic radiation is useful in chemistry as it can be both absorbed or emitted. When agnetic radiation is absorbed or emitted it behaves like a stream of particles (so we have to uses the particle model rather than the wave model). These particles are called photons. When a photon is emitted or absorbed, energy is lost or gained by the electrons in the sample being studied. Photons which are at high frequencies transfer larger amounts of energy than photons in lower frequencies. The relationship between the energy (E) carried by a photon and its frequency is given by: E = hf h is Planck's Constant = 6.63 x 10-34 Js. This refers to 1 photon of energy emitted or absorbed when 1 electron moves energy level. 1a) Electromagnetic Radiation and Atomic Spectra 2 For one mole of photons the relationship is: E = Lhf Therefore it can be deduced that: E Lhe Example Calculate the wavelength of light corresponding to the bond energy of Cl - Cl (in nanometres) Lhc E= X Lhc E λ = A = ((6.02×10²³)×(6.63×10−³¹)×(3.00×10³) (243x1000) A = 4.9 x 10-7m x 10⁹ X = 492.7nm x1000 to convert to Joules L is Avogadro's Constant = 6.02 × 1023. Electrons Electrons are arranged in energy levels. If an electron has enough energy, these electrons can move from a lower energy level to a higher energy level. Each electron in an atom has a fixed amount of energy depending on the energy level to which it belongs. When electrons absorb or emit energy and move to a higher level, they are said to be in an excited state. When excited electrons drop back down to their ground state, energy is released in the form of a photon. The photons released fall within the visible spectrum. Atomic Emission Spectra 1a) Electromagnetic Radiation and Atomic Spectra 3 When a beam of white light is passed through a prism or from a diffraction grating onto a screen, a continuous spectrum is visible. When white light from a source passes through a sample being heated, the spectrum turns out not to be a continuous spectrum, but a series of lines of different wavelengths and thus of different colours. Each line corresponds to the energy given out when excited electrons move down to a lower energy level, generating photons of various frequencies. Theses lines correspond to certain specific frequencies and wavelengths found in the visible or ultraviolet spectrum. Each element creates its own unique spectrum with its own specific frequencies and wavelengths. White light source Sodium chloride sample in flame White light source Sodium vapour lamp BB Slits THEE Prism 1a) Electromagnetic Radiation and Atomic Spectra Screen (a) Continuous spectrum (b) Line emission spectrum types of spectrum (c) Line absorption spectrum Atomic Absorption Spectra When a beam of continuous radiation like white light is directed through a gaseous sample, it can cause an atom to make a transition from its ground state to an excited state. If the frequency of the light, and therefore the energy of the photon, corresponds to an excitation energy of the atom, then the photon of light is absorbed. The radiation that emerges will therefore have certain wavelengths missing. These show up as dark lines on a continuous spectrum called an atomic absorption spectrum. Using Spectra to Identify Samples Both emission and absorption spectroscopy can be used to determine whether a certain species is present in a sample and how much of it is present, since the intensity of the transmitted or absorbed radiation can be measured. 4 A calibration graph is first made using known concentrations of the species being analysed. The radiation absorbed or emitted by the species in these samples is then plotted against concentration. When the unknown sample is analysed, the concentration of the species can be found from the graph by reading off the concentration of the known sample with the same absorbance. Absorption % Calibration graph Concentration /mol μ-¹ Unknown sample 16% IK 0.1 mol 1-¹ Concentration/mol H analysing an unknown concentration by spectroscopy 1a) Electromagnetic Radiation and Atomic Spectra 5