Advanced Higher Advanced Higher ChemistryChemistry

Unit 1 Unit 1

SpectroscopySpectroscopy

SpectroscopySpectroscopy

Spectroscopy is used to give information Spectroscopy is used to give information regarding the structure of atoms or molecules.regarding the structure of atoms or molecules.

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

Spectroscopy Spectroscopy

Atomic Emission Spectroscopy (AES) or Atomic Emission Spectroscopy (AES) or Atomic Absorption Spectroscopy (AAS) Atomic Absorption Spectroscopy (AAS) involve the transition of electrons between involve the transition of electrons between energy levels in atoms.energy levels in atoms.

The energy differences usually correspond The energy differences usually correspond to the visible region (400 – 700nm) but to the visible region (400 – 700nm) but some applications use the UV region some applications use the UV region

(200 – 400nm).(200 – 400nm).

Excited AtomsExcited Atoms

• When atoms absorb When atoms absorb energy, they become energy, they become excited.excited.

• Electrons move to a higher Electrons move to a higher energy. When they fall back energy. When they fall back down to ground state, a down to ground state, a photon of light is emitted.photon of light is emitted.

Emission Spectra and EnergyEmission Spectra and Energy

• The difference between energy levels is The difference between energy levels is fixed.fixed.

• When an electron falls back to ground When an electron falls back to ground state, the wavelength of light emitted can state, the wavelength of light emitted can be used to calculate the difference in be used to calculate the difference in energy. energy.

Emission SpectraEmission Spectra

• Elements have their own characteristic Elements have their own characteristic emission spectrum.emission spectrum.

• For example, the emission spectrum for For example, the emission spectrum for neon has lines between 580 and 670 neon has lines between 580 and 670 nm (the red region of the visible nm (the red region of the visible spectrum) therefore glowing neon spectrum) therefore glowing neon appears red.appears red.

Atomic Absorption SpectroscopyAtomic Absorption Spectroscopy

The sample to be tested is illuminated with The sample to be tested is illuminated with a range of frequencies.a range of frequencies.

If a photon of light hits an atom it may If a photon of light hits an atom it may cause the atom to make a transition from cause the atom to make a transition from lower to higher energy levels.lower to higher energy levels.

If the light’s frequency does not match to a If the light’s frequency does not match to a higher energy state of the atom then the higher energy state of the atom then the photon is not absorbed.photon is not absorbed.

Atomic Absorption SpectroscopyAtomic Absorption Spectroscopy

The radiation that has been passed through the The radiation that has been passed through the sample is then passed through a diffraction sample is then passed through a diffraction grating. The frequencies absorbed appear as grating. The frequencies absorbed appear as black lines against a spectrum of colours.black lines against a spectrum of colours.

Because each element has a characteristic Because each element has a characteristic spectrum it can then be identified.spectrum it can then be identified.

The frequencies of dark lines in an absorption The frequencies of dark lines in an absorption spectrum will be identical to the coloured lines in spectrum will be identical to the coloured lines in an emission spectrum since they represent an emission spectrum since they represent transitions in opposite directions between the transitions in opposite directions between the same energy levels.same energy levels.

Atomic Emission SpectroscopyAtomic Emission Spectroscopy

• By analysing the wavelengths of the light By analysing the wavelengths of the light emitted, AES can determine which emitted, AES can determine which elements are present in the sample.elements are present in the sample.

By measuring the intensity of the light By measuring the intensity of the light emitted, AES can also determine the emitted, AES can also determine the quantity of the element in the sample.quantity of the element in the sample.

Uses of emission and absorption Uses of emission and absorption spectroscopyspectroscopy

Emission spectroscopy is used in flame Emission spectroscopy is used in flame testing for testing group 1 and 2 metals.testing for testing group 1 and 2 metals.

Sodium vapour and mercury vapour street Sodium vapour and mercury vapour street lights and neon signs have specific colours lights and neon signs have specific colours due to their emission spectra.due to their emission spectra.

Both emission and absorption spectra are Both emission and absorption spectra are used to detect trace elements in water and used to detect trace elements in water and food samples and in the analysis of food samples and in the analysis of industrial materials e.g. steels.industrial materials e.g. steels.