Edexcel A Level Chemistry:复习笔记1.2.2 Quantum Shells

Developing Electronic Configuration

 

  • 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:

 

2.1.5-Absorption-and-emission 

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

2.1.5-Line-emission-visible-spectrum-of-hydrogen

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-of-Calcium-

Successive ionisation energies for the element calcium

 

Filling Quantum Shells

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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