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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