Valence electrons are the electrons that are
farthest from the nucleus.
These are the electrons in the highest occupied
shell.
These electrons are chemically reactive.
Looking at the electron configurations on
table two point three from the text we see
that the highest shell for beryllium, for
example, is two and there are two electrons
in the two s subshell.
Thus beryllium has two valence electrons.
The highest shell for neon is also the second
shell.
However, neon has two electrons in the two
s subshell and six electrons in the two p
subshell.
Thus neon has eight valence electrons.
The highest shell for sulfur is three.
Since sulfur has two electrons in the three
s subshell and four electrons in the three
p subshell, sulfur has six valence electrons.
You can always find the valence electrons
from an electron configuration, however there
is an easier way to determine the number of
valence electrons in an atom.
We will only need to know the valence electrons
of main group elements – which are the elements
in the s and p blocks.
To determine the number of valence electrons
in a main group element you merely count columns
from left to right, ignoring the d block if
the element is in the p block.
For example, magnesium is in the second column
and thus has two valence electrons.
Any member of the alkaline earth metal family
will have two valence electrons.
Boron has one, two, three valence electrons
since it is in the third column, ignoring
the d block.
Any member of the boron family has three valence
electrons.
The only exception to this is helium.
One, two, three, four, five, six, seven, eight.
All of the noble gases have eight valence
electrons except helium.
Helium only has two electrons, so it cannot
have eight valence electrons.
The family number with the A in it also gives
the number of valence electrons in a family
– except for helium.
For example, oxygen, sulfur, selenium, tellurium
and polonium are all in family six A. These
elements all have six valence electrons.
