In this lesson, we're going to look at
the difference between valence and core
electrons, we're going to learn to
identify the number of valence electrons
in an element, and understand how atoms
become more stable because of the octet
rule. Let's look at an example with
aluminum. How many electrons are in a
single atom of neutral aluminum? We have
an atomic number of 13 which means 13
protons, so we must also have 13
electrons. Next, we're going to arrange
our electrons in the different energy
levels. So we have 2 in the n=1
level, 8 in the n=2 level, and 3 in the n=3
level. When we look at this, what we see
is that these are the core electrons in
the n=1 and n=2 level and
the top electrons in the n=3
level are what we call our valence
electrons. So the valence electrons are
those which are in the highest energy
level where there are electrons. Notice
I'm talking about then=3 level
here. There are no electrons in the n=4
level, so that won't be the
valence shell. The n=3 will be
the valence shell. Anything in a lower
energy level will always be the core
electrons. The reason we're so concerned
about the valence electrons is because
these are the electrons that are
involved in bonding. Whether we're
talking about forming molecular
compounds or ionic compounds, it's the
valence electrons that undergo change
and interact with other elements to form
bonds.
Let's look at some trends in electron
arrangements that are going to help us
predict the number of valence electrons
in many of the elements on the periodic
table. Let's start with lithium. In
lithium, we have three electrons which
means we'll have two electrons in the n=1 level and one electron in the n=2
level. When we go to sodium, we
have 11 electrons total. We have two in
the n=1, eight in the n=2, and one in n=3. When we go
on to potassium, we have 19 electrons so
we have two, eight, eight, and one. Notice
that all three of these have one
electron in their highest occupied
energy level and therefore, they all have
one valence electron. When we look at the
periodic table, we'll see that lithium,
sodium, and potassium are all in the same
column of the periodic table. As a result,
we know that elements in that first
column of the periodic table will all
have one valence electron. Now let's look
at the second column. We have beryllium
with four electrons so we have two and
two. Magnesium has 12 so we have two,
eight, and two. Calcium has 20 so we have
two, eight, eight, and two and we see that
all three of these elements have two
valence electrons. All three of these
elements are in the second column of the
periodic table. Let's look at one more
example on the right side of the
periodic table. Looking at fluorine,
chlorine, and bromine these are in a
group known as the halogens which are in
the second to last column right next to
the noble gases. We see that we have
9 electrons in fluorine so we have 2 &
7, we have 17 electrons in chlorine so 2
& 8 and then 7, and in bromine we have 35
electrons. Now bromine is past where we
are going to look at determining the
electron arrangements, but what we do
know is
that bromine also has seven electrons in
then=4 shell. What we can see
is that all of these elements have seven
valence electrons. So knowing these
trends will help us predict the number
of valence electron of many of our
elements even if we just know the number
of valence electrons in the element at
the top of the column. When we look at
our noble gases which are in the last
column on the right side of the periodic
table, what we see is that these elements
are very unreactive. They're much less
likely to interact with other atoms to
form bonds and when we look at our
electron arrangement for one of them,
neon, what we see is that we have ten
electrons so that means we have two
electrons and eight electrons and the
two energy levels. If we did this for
all of our noble gases, what we would see
is that we have a similar pattern. They
have eight valence electrons and this is
where the octet rule comes into play. So
the noble gases have eight valence
electrons which gives them a great
amount of stability so they don't need
to bond in order to get more electrons
around them. When we start looking at
when elements form charged species, we're
going to see how they gain or lose
electrons in order to look more like the
noble gases. So if we look at oxygen, it
has eight electrons so two and six,
fluorine has two and seven, sodium has 11
total with two, eight, and one, and
magnesium with two, eight, and two. Looking at these values, we're going to
see that oxygen and fluorine would like
to gain two and one electrons
respectively in order to look like neon. Sodium and magnesium will want to lose
one and two electrons in order to look
like neon.
