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Electron Configurations

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Write electron configurations like 2,8,1 for the first 20 elements and their ions, then use them to find groups and periods. Answer correctly to see the shell diagram.

The rules
  1. Electrons = atomic number in an atom. Count them all before you start.
  2. Fill the shells from the inside out. The first shell holds up to 2 electrons. The second and third shells hold up to 8 each (for the first 20 elements).
  3. Potassium and calcium start a fourth shell after the third shell has 8: potassium is 2,8,8,1 and calcium is 2,8,8,2.
  4. Positive ions have lost electrons. Negative ions have gained electrons. Take away or add the charge, then fill the shells again.
  5. Group = electrons in the outer shell, except that a full outer shell is group 0 (the noble gases).
  6. Period = number of occupied shells.
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Worked solution

    The 2,8,8 rule works for the first 20 elements, which is what GCSE needs. Diagrams show electrons as crosses. Electrons gained by a negative ion are shown as dots.

    Type configurations with commas, full stops or spaces between the shells, for example 2,8,1 or 2.8.1 or 2 8 1. For an element, type its name or symbol.

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