• Electrons move rapidly around the nucleus in energy shells
• If their energy is increased, then they can jump to a higher energy level
• The process is reversible, so electrons can return to their original energy levels
  • When this happens, they emit energy

   

• The frequency of energy is exactly the same, it is just being emitted rather than absorbed:

The difference between absorption and emission depends on whether electrons are jumping from lower to higher energy levels or the other way around

• The energy they emit is a mixture of different frequencies
• This is thought to correspond to the many possibilities of electron jumps between energy shells
• If the emitted energy is in the visible region, it can be analysed by passing it through a diffraction grating
• The result is a?line emission spectrum

Line emission spectra

The line emission (visible) spectrum of hydrogen

• Each line is a specific energy value
  • This suggests that electrons can only possess a limited choice of allowed energies

   

• These packets of energy are called 'quanta' (plural?quantum)
• What you should notice about this spectrum is that the lines get closer together towards the blue end of the spectrum
• This is called?convergence?and the set of lines is?converging?towards the higher energy end, so the electron is reaching a maximum amount of energy
• This maximum corresponds to the?ionisation energy?of the electron
• These lines were first observed by the Swiss school teacher Johannes?Balmer, and they are named after him
• We now know that these lines correspond to the electron jumping from higher levels down to the?second?or?n = 2?energy level

Successive ionisation energies

• The?first?electron removed has a low?IE1?as it is easily removed from the atom due to the spin-pair repulsion of the electrons in the 4s orbital
• The?second?electron is more difficult to remove than the first electron as there is no?spin-pair repulsion
• The?third?electron is much more difficult to remove than the second one corresponding to the fact that the third electron is in a?principal quantum?shell which is closer to the nucleus (3p)
• Removal of the?fourth?electron is more difficult as the orbital is no longer full, and there is less?spin-pair repulsion
• The graph shows there is a large increase in successive ionisation energy as the electrons are being removed from an increasingly positive ion
• The big jumps on the graph show the change of?shell?and the small jumps are the change of?subshell

Successive ionisation energies for the element calcium

Shells

• The arrangement of electrons in an atom is called the?electron?configuration
• Electrons are arranged around the nucleus in?principal energy levels?or?principal quantum shells
• Principal quantum numbers?(n) are used to number the energy levels or quantum shells
  • The?lower?the principal quantum number, the closer the shell is to the nucleus
    • So, the first shell which is the closest to the nucleus is n = 1

     

  • The?higher?the principal quantum number, the greater the energy of the shell and the further away from the nucleus

   

• Each principal quantum number has a?fixed?number of electrons it can hold
  • n = 1 : up to 2 electrons
  • n = 2 : up to 8 electrons
  • n = 3 : up to 18 electrons
  • n = 4 : up to 32 electrons

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