Quantum - Line spectra

• The energy levels are quantised, meaning that electrons can only exist at certain energies.
• Electron transitions between different energy states result in the emission or absorption of photons with specific wavelengths.
• The frequency of light emitted by an atom is determined by its electron configuration.
• Define ionisation energy

  The minimum energy needed to remove an electron from the ground state to be completely free of the atom.
• What is the lowest state in which electrons can exist?

  The ground state
• Complete


  If an electron is at a level higher than n=1 (the ground state) it is said to be excited.
• Complete


  Because electrons have to fall between discrete energy levels, only certain discrete energy photons are emitted and thus have discrete frequencies hence lead to a line spectra from certain gases corresponding to these transitions.
• What is excitation?


  When an electron has been promoted to a higher energy level than its ground state.
• What is ionisation?


  When an electron leaves the atom entirely.
• Electron collisions - When a bombarding electron collides with a gas atom, what are the three things that can happen?

  1. An elastic collision
  2. An inelastic collision
  3. Ionisation
• What happens in an elastic collision?


  The bombarding electron gives a small amount of kinetic energy to the atom. Kinetic energy is conserved in an elastic collision.
• What happens in an inelastic collision?

  An orbiting atomic electron gains exactly the amount of energy it requires to reach a higher energy level. The energy lost by the bombarding electron is released as electromagnetic radiation a short time after, when the atom returns to ground state.
• What can also occur in an inelastic collision?

  Ionisation
• What is excitation potential?

  The voltage through which electrons need to be accelerated in order to promote ('atomic') electrons to a higher energy level. This is numerically equivalent to the excitation energy in
• What is ionisation potential?

  The voltage through which electrons need to be accelerated in order to promote ('atomic') electrons to zero energy level and hence escape from the atom. This is numerically equal to the ionisation energy in eV.
• Line spectra?


  Line spectra are only produced by low density monatomic gases and vapours. Typical sources are low pressure discharge tubes e.g. sodium lamps.

  The lamps emit light due to electrons being excited from one energy level to a higher one.

  When the electrons return to a more stable state they emit energy of a specific frequency/wavelength depending on the difference between the two energy levels.

  Each element has a characteristic set of energy levels so each element will have a characteristic line spectra by which it can be identified.
• Continuous spectra?

  This is a spectrum of all frequencies without any 'gaps'.

  They are produced by hot solids & liquids and high density gases.

  The atoms are close together and interact with each other so the electrons have a continuous range of energies and so produce radiation of all frequencies/wavelengths
• Absorption spectra

  When an electron in a low energy level absorbs a photon of exactly the right energy, it can be raised to a higher energy level
• The photon must have an energy equal to the difference between the two energy levels
• The electron can be raised to other energy levels by different energies of photons, but the photons must have an energy equal to the difference between the two levels
• Absorption spectrum formation

  1. Light hits a gas

  2. Certain wavelengths, which correspond to certain energies, are absorbed

  3. A continuous spectrum is seen with dark lines where the photons have been absorbed
• What type of spectrum does a filament lamp produce?

  A continuous spectrum
• What does a fluorescent tube contain?

  Mercury and phosphor coating.
• What happens in a fluorescent tube?

  Electrons accelerated along the tube collide with mercury atoms and excite them causing their orbiting electrons to be promoted to a higher energy level. When these drop back down, most of the photons emitted are in the ultra violet part of the spectrum. When the ultra violet photons are absorbed by the phosphor coating, the phosphor atoms are excited and they emit photons over a larger range of wavelengths in the visible part of the spectrum.