So of course we can't leave the periodic table
out of the conversation when we're talking chemistry.
But thankfully, as biologist we only have to
look at the first three periods and these first
three periods correspond with three energy levels that we've
talked about previously. Lets look at carbon
for example. Carbon is atomic number 6 which means
by definition it has 6 protons. We also see
that it has an atomic mass of 12. This atomic
mass tells us that there are also 6 neutrons.
Each of those has an atomic mass of 1, 6+6 is 12.
So now, lets look at how we can tell from looking
at any atom in the periodic table what its
electron arrangement might be in the energy shells.
Here we're looking at the second period.
Carbon is the second row down in the table.
It's the second period. This means the carbon has
two electron shells. The first shell is full
with two electrons. The second shell is
going to be full with 8 but it only has 4.
Hence atomic number 6. Also we can see that carbon
is in the fourth column. Again, we only need to look
at the blue part. In the second period meaning
we have 2 electron shells and 4 spots over
meaning that there are 4 electrons in that
outermost shell presently. Now how would carbon
be most comfortable. The octet rule applies here
to say that carbon would probably be most comfortable
if it had its outer electron shell full. In which
case it has the potential to get 4 more electrons
in its outer shell. So in biology, there are only
six major elements that we need to know as well as
some electrolytes like potassium and calcium
and sodium. But the main 6 elements are called
the SPONCH elements. For ease of memory, SPONCH
makes complete sense. We have sulfur, we have phosphorus,
we have oxygen, nitrogen, carbon, the cornerstone
of biological molecules and then hydrogen.
So once again luckily we only have to deal with
a certain smaller portion of chemistry in our
study of biology. But our understanding of those
atoms is pretty important. So in order to consider
bonding arrangements, we really are concerned
mostly with the outermost shell. We call this
the valence shell. The number of electrons in the
valence shell is how many valence electrons we have.
We know that the first shell can carry two
electrons. So that's illustrated in period 1.
Period 2 shows us that the second shell can be
filled with 8 electrons and period 3 also can
be filled with 8 electrons. In order to fulfill
the octet rule, an atom is looking to be happy
having 8 electrons in its outermost shell.
How many electrons that an atom would like to have
is considered to be its valence. So we say that
an atoms valence is how many electrons it would need
to satisfy the octet rule. So when an atoms valence
shell is full, its non-reactive because it doesn't
need to find anymore electrons to fill that shell.
For example here, helium has its innermost shell
its only shell full with two electrons. So lets
consider nitrogen. How many valence electrons
does it have? We can count the electrons in the
outermost shell. We know that it has 5 electrons
in its outermost shell. But lets contrast that
to the question of what is the valence of nitrogen?
The valence again is how many would it like to
satisfy the octet rule. In this case, it would like
3 in order to satisfy that octet rule. So it has
a valence of 3. It would like to find 3 more electrons.
What about helium? It has an outer shell, one shell.
It's the innermost shell. It's full with 2 electrons.
So it has no valence. It has no desire to bond
or associate with other atoms. From this lecture,
you should have gained a great understanding of
atomic structure. At this point, you should be
able to diagram an atom including the protons,
neutrons and electron arrangements.
In addition, you should have a great understanding
of what valence means so that in the next lecture
we can move on and explore the chemical
foundations of biology a little further.