Valence Electrons

Valence electrons are the electrons that exist on the outermost shell of an atom.
Are the ones involved in chemical reactions

An open shell is a shell that has less than the maximum amount of electrons in it.
A closed shell is a shell that is completely full with the max. amount of electrons that can occupy it.

How to determine the number of valence electrons from electronic configurations:

Sodium: has 11 electrons.

1s^2 2s^2 2p^6 3s^1

Now we must put it into core notation:
[Ne] 3s^1

When counting valence electrons, only the s and p subshells are considered, unless the d and f subshells are full.

Therefore, Na has one valence electron.

http://www.youtube.com/watch?v=1TZA171yxY4&feature=fvwrel

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