Electronic Structure 

The fact that electrons occupy ‘shells’ around the nucleus is what causes the whole of chemistry.

Electron shell rules:

  • Electrons always occupy shells(sometimes called energy levels).
  • The lowest energy levels are always filled first-these are the ones closest to the nucleus
  • Only a certain number of electrons are allowed in each shell:   * 1st shell = 2   * 2nd shell = 8   * 3rd shell = 8
  • Atoms are much happier when they have full electron shells-like the noble gases in Group 0
  • In most atoms, the outer shell is not full and this makes the atom want to react to fill it

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Follow the rules to work out electronic structures

  • You can easily work out the electronic structures for the first 20 elements of the periodic table(things get a bit more complicated after that)   * Example: What is the electronic structure of nitrogen?     * Nitrogen’s atomic number is 7. This means it has 7 protons, so it must have 7 electrons     * Follow the electron shell rules above. The first shell can only take 2 electrons and the second shell can take a maximum of 8 electrons     * So the electronic structure for nitrogen must be 2,5

  • More examples:   * Hydrogen = 1   * Helium = 2   * Lithium = 3   * Carbon = 6   * Neon = 10   * Calcium = 20

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