• Paul Morgan

A Level Chemistry Energy Level Filling

Updated: Aug 26, 2019

Electron Energy Level Filling

Atoms

Remember for atoms fill the 4s before 3d...

1s2s2p3s3p4s3d..(In order of increasing energy)

So Calcium (20 electrons)...

1s2 2s2 2p6 3s2 3p6 4s2

And Scandium (21 electrons)

1s2 2s2 2p6 3s2 3p6 4s2 3d1

What about ions?

There are two ways of looking at this.

Option 1)

Although we fill the 4s before the 3d sub level we also empty the 4s sub level before the 3d. So..

Ca atom... 1s2 2s2 2p6 3s2 3p6 4s2

Ca 2+ ion (loses 2 highest energy electrons - 4s2) 1s2 2s2 2p6 3s2 3p6

And Sc atom... 1s2 2s2 2p6 3s2 3p6 4s2 3d1

Sc 3+ (loses 3 highest energy electrons 4s2 and 3d1)

So Sc 3+ is..

1s2 2s2 2p6 3s2 3p6

Cr and Cu are exceptions to this rule which we'll have a look at in the future.

Option 2)

For ions count the total number of electrons left after ionisation then fill the 3d before the 4s and you'll end up with the same electron arrangement as above. Try it.

Sc3+ has 18 electrons therefore just fill energy levels in number order..

1s2 2s2 2p6 3s2 3p6

Ca2+ has 18 electrons therefore also...

1s2 2s2 2p6 3s2 3p6

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