Electron Shells

• Electrons occupy shells(sometimes called energy levels)
• The lowest energy levels are always filled first
• 1st shell is allowed ? electrons

  2 electrons
• 2nd shell is allowed ? electrons


  8 electrons
• 3rd shell is allowed ? electrons


  8 electrons
• The periodic table tells you that nitrogen has 7 protons, so it must have 7 electrons.
• The number of shells which contain electrons, is the same as the period of the element
• The group number tells you how many electrons occupy the outer shell of the element.
• Sodium is in period 3 so has ? shells occupied.

  3 shells occupied (so the first two shells must be full)
• Sodium is in group 1, so it has ? electron in its outer shell. It's electronic structure is ?.?.?


  Sodium has 1 electron in its outer shell. It's electronic structure is 2.8.1