Arrangement atom

• Spectra

  An array of colours formed when white light is passed through a prism
• Continuous spectrum

  • All possible colours are present
• Line spectrum

  • A series of narrow coloured lines
• Emission line spectrum is formed when light from a hydrogen discharge tube is passed through a prism
• Each element has its own unique emission line spectrum
• Spectrometer


  Instrument used to carry out measurements on spectra
• Spectroscope

  Instrument used to observe spectra
• Flame tests can be used to identify metals based on the colours they emit
• Colours emitted by salts of various metals

  • Crimson (Lithium)
  • Lilac (Potassium)
  • Green (Barium)
  • Red (Strontium)
  • Blue-green (Copper)
  • Yellow (Sodium)
• The colours in fireworks displays are obtained by adding salts of various metals to the explosive mixture
• Quantisation of energy

  An electron in an atom can only have certain fixed amounts of energy, not any value
• Bohr's theory of atomic structure

  • Electrons revolve around the nucleus in fixed paths called orbits/energy levels
  • The energy of an electron in an energy level is quantised, i.e. fixed at a definite value
  • Atoms normally exist in the ground state with electrons occupying the lowest available energy levels
  • When energy is absorbed, electrons jump to higher energy levels (excited state)
  • As electrons fall back to lower energy levels, light of a definite frequency is emitted
• The frequency of the emitted light is proportional to the energy difference between the two energy levels, as given by the equation E2-E1=hf
• The definite amount of energy emitted (light of a definite frequency) appears as a line of a particular colour in the emission spectrum
• Each element has its own unique arrangement of energy levels, resulting in its own unique emission spectrum
• Each element has its own unique distribution of energy levels
• Energy is given out in the form of light of a definite frequency when an electron falls back from an excited state to the ground state
• The frequency of the light is proportional to the energy difference between the two energy levels and may be calculated from the equation E₂-E₁=hf
• Since only definite amounts of energy are emitted, this implies that electrons can occupy only definite energy levels
• Each definite amount of energy emitted gives rise to a line in the emission spectrum
• Series of electron transitions

  • Balmer series
  • Lyman series
  • Paschen series
• Bohr's theory correctly predicted the wavelengths of light in the emission line spectrum of the hydrogen atom
• The fact that only definite amounts of energy are emitted implies that electrons can only occupy definite energy levels, i.e. Bohr's suggestion that energy levels must exist in the atom was proven correct
• Even in a small sample of hydrogen, there are millions of atoms, and not all atoms may receive exactly the same amount of energy, so different electron transitions are possible
• Energy levels

  The fixed energy values that electrons in an atom may have
• Colours imparted to Bunsen flame by different metals

  • Yellow (sodium)
  • Lilac (potassium)
  • Green (barium)
  • Red (strontium)
  • Blue-green (copper)
  • Crimson (lithium)
• Atoms in the ground state absorb the same radiations as they emit in the excited state
• Absorption spectrum

  Dark lines against a coloured background, like a photographic negative of an emission spectrum
• Emission spectrum

  Coloured lines against a dark background
• Atomic absorption spectrometry

  • Useful instrumental technique for detecting the presence of certain elements in materials dissolved in water and for measuring the concentrations of these elements
  • Uses lamps that emit the line spectrum of the element being analysed, as the light sources produce exactly the same wavelengths that will be absorbed by the sample
• Atomic absorption spectrometry is used in forensic science to test for gunshot residue
• Energy sublevels


  Each main energy level, except the first, is made up of a number of sublevels all of which are close in energy. The number of sublevels is the same as the value of n for the main energy level.
• Labelling of sublevels


  • s
  • p
  • d
  • f
• Flame atomic absorption spectroscopy

  1. Use same light sources as elements being analysed

  2. Hold sample containing sodium in flame

  3. Measure intensity of light transmitted through flame before and after sample introduced

  4. Instrument indicates amount of light absorbed by sample

  5. Measure concentration of sodium
• The light sources produce exactly the same wavelengths as those that will be absorbed by the sample
• From the amount of light absorbed, it is possible to measure the concentration of sodium
• Main energy levels

  Labelled with numbers (1, 2, 3, etc.)
• Sublevels


  Subdivisions of main energy levels, labelled with letters (s, p, d, f)
• The s sublevel has the lowest energy, followed by p, d, and f
• The 4s sublevel is lower in energy than the 3d sublevel