The valence shell is the outermost (highest
energy)
shell in which a given atom has
electrons.
Electrons surround
atomic nuclei in
probability density patterns called
orbitals. Lower energy
orbitals are tighter around the
nucleus;
electrons are
attracted to
protons, so it takes less
energy for them to maintain a short distance from the nucleus. The valence shell
electrons are most likely to be used in a
molecular bond because, being in the highest energy
shell, they are least firmly held by the
nucleus.
Electrons fill
orbitals in an order easily determined from the following chart.
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d
7s 7p
8s
Draw lines through each top right-lower left diagonal, starting with 1s, to get the following order:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p ...
The s
orbitals hold 2
electrons each, the p's 6, the d's 10, the f's 14. Going past f is unnecessary; we can't make
atoms that
gargantuan.
Having placed all the
atom's
electrons, count back until you reach the beginning of an s
orbital and you'll have the number of
valence electrons.
Valence electrons don't really apply to
atoms with d and f
orbitals. They swap
electrons around between
orbitals of roughly equivalent
energies. Working with them in this context can give you "the
crazy" if you attempt it unprepared, but fear not, said preparation will eventually be provided.
Because you'll mostly only work with s and p
orbitals,
atoms will seem to follow an "
octet rule" and attempt to share
valence electrons in such a way as to have eight shared
valence electrons, the sum of the number of
electrons in an s and a p
orbital pair.