Electron Shells

OK, this page basically explains the whole of chemistry. This doesn’t mean you can get away with just revising this one page though….sorry about that.

So here is the big secret of chemistry: ELECTRONS OCCUPY “SHELLS” AROUND THE NUCLEUS.

There’s a few more rules to it than that though:

  • Electrons occupy “shells” or “energy levels” around the nucleus.
  • The lowest levels are the ones closest to the nucleus and these are the ones that are always filled first.
  • Only a certain number of electrons are allowed in each level:

1st Shell – 1, 2nd Shell – 8, 3rd Shell – 8

  • Atoms like their shells to be filled. If they have a full level, like the noble gasses they won’t react….if they don’t have a full outer level though they will react with other atoms to try to fill it.

Working Out Electronic Structures

You need to be able to work out the electronic structures – i.e. how many electrons are in each level – for the first 2 elements.

It’s really easy, just use the following method:

  • Look at the periodic table to see the number of neutrons an element has (usually the number in the bottom left)
  • The number of electrons is equal to this number of protons.
  • Follow the rules above about the number of electrons in each shell
  • Write your answer as numbers with commas between them – e.g. 2,8,4

So let’s have a look for Chlorine. See if you can work it out, then click below to see if you were right.

chlorine atomic number

[accordion][item title=”Click to reveal answer”]If we look at the periodic table we see chlorine has 17 protons, so it must have 17 electrons. This means it’s electronic structure is 2,8,7.[/item]
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NOTE: If asked to draw a diagram just do something like this:

chlorine atom