Predicting Valence Electrons

Valence electrons are located in the outermost open shell of an electron. These take part in chemical reactions.

What we mean by open shell is that the shell contains less than the maximum number of electrons.
In contrast, a closed shell would be completely full.



The valence electrons are all the electrons in the atom except in core and filled d/f subshells. In other words, the valence electrons are the s, p types of the outer set, and ONLY unfilled d and f subshells.

For example, if we were given the configuration of Calcium as:

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

To see the valence electrons easier, put that configuration in Core Notation:

[Ar] 4s2

We see there are only 2 electrons in the outer set, and they meet the rules, since it is an s-subshell.

For a harder one, let's do Pb.

[Xe] 6s2 4f14 5d10 6p2

How many valence electrons are there?

If you got 4, you are correct. This is because there are 2 from the s-subshell, and 2 from the p-subshell. We do not count the f and d, because they are FILLED.

And that is all for this short section! :)