Hey folks, this is Boghossian. This
screencast is going to be for you folks
who did not take Chem300, or maybe just
need a little refresher on chemistry.
The things that we're going to cover in
this screencast; we're going to do a
really brief review of the periodic
table, a little bit about subatomic
particles, something called the octet
rule, and then the basics of chemical
bonding. You've probably seen this
table before; this is called the periodic
table of elements, and this demonstrates
how we keep all of the elements that we
have in the world organized. Each one of
these little squares actually has a
pretty sizable amount of information
about each different element, so we're
gonna look at how to read those. The
first element that we're going to look
at is going to be this one right here,
which is sodium. There's actually a lot
of information contained in this little
box right here. Every box has the name
of the element written in them in full,
and then the box also has a chemical
symbol, that represents each element;
some of their names are really
long and it would be really challenging
to write them out every single time you
want to draw a diagram. Then, this number
down here at the bottom is called the
atomic mass, but we really won't use a
whole lot of that in biology. The most
important piece of information for our
purposes is going to be the number in
the upper left corner of any of these
boxes, and that's called the atomic
number. It's going to tell us a lot about
how the element is going to interact
with other elements. This number is
going to tell us a lot about how it
interacts because it's going to tell us
about the numbers of different subatomic
particles that are contained within one
atom of each element.
Let's take a closer look at sodium. So,
you may remember that there are three
different components to atoms and they are
neutrons, protons, and electrons, and each
one of them has a different role in
making up the atom. Neutrons have no
charge, and protons have a positive
charge, and both of those are found
within the nucleus of the atom. Sodium
has an atomic number of 11, which means
that its nucleus would usually contain
11 protons and 11 neutrons, so look kind of
like this. This cluster of neutrons
and protons is what we call the nucleus of
the atom.
Electrons are also a key part of the
atom, because they have a negative charge,
but they are not contained within the
nucleus. Most of the time, sodium is
going to have 11 electrons, and they're
arranged in layers around the outside of
the nucleus. Each of these layers is
called an orbital, and this first layer
is called the s-orbital, and it can hold
two electrons. However, sodium doesn't
only have two electrons, it has 11
electrons, and so we're going to need to
find a place to put the rest of those
electrons. We're going to add
another orbital around the outside like
this, and this orbital is going to be
called the p orbital. This second orbital
or shell of electrons can actually hold
eight electrons. So, so far we held two in
the first layer, and then another eight
in the second layer, which is a total of
ten. But sodium still has 11 electrons.
and so we need to find a place to put
the final one.
Our outermost shell is going to
contain that final electron. So far, we
have diagrammed all of our neutrons and
protons, and then 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
electrons in three layers. What's
going to be the most important is going
to be this layer the outermost layer
because the outermost orbital of
electrons going to enable the atom to
interact with other atoms. So, let's go
back to our periodic table for a minute.
Here's sodium right here, and if we
remember it had one electron in the
outermost shell. We call this outermost
electron a valence electron, and
actually everything else in this column
right here also has one valence electron.
So, hydrogen, lithium, sodium, potassium all
the way down in that column they all
have one valence electron. Now we're
back to our sodium atom; it has one
valence electron, but it has seven empty
spaces in this shell. So, one here, here
here, here, here, here, and here. It really
wants to have eight electrons, in the
outermost shell, and we call this the
octet rule. So, in order to satisfy the
octet rule, this sodium atom now has two
options;
it could get rid of this electron right
here and thus have eight electrons in
this outer shell, or if it didn't want to
do that and decided they wanted to do something
else it could actually try to acquire
seven other electrons to fill all of
these spaces right here. Now, one of these
options is going to require a pretty small
amount of effort, and
the other option would require a lot of
effort, and so let's take an educated
guess as to which one is probably going
to be chosen by the sodium atom. So, if
you guessed that it's probably going to
want to jettison this electron right
here in order to satisfy the octet rule,
you are correct. This is exactly what's
going to happen; it's going to get rid of
this electron right here. So, let's figure
out what the impact of jettisoning this
electron would be. Let's figure out what
the charge of this atom is before we
start. Because neutrons have a zero
charge, we don't have to worry about them,
let's count up the number of protons
and electrons. Currently we have 11
protons, and each one of them has a
charge of positive one,
we also have 11 electrons in each one of
them has a charge of negative 1, which
means that are positive and negative
charges in this atom currently balance
each other out perfectly. And so, we have
a net charge of zero. This atom is really
unhappy in this state, however, because it
doesn't have a full outer shell; it
hasn't satisfied the octet rule in its
current form. Let's see what happens if
we put this sodium atom in the same
vicinity as a chlorine atom. Now, if you
look at these two atoms that we have
drawn here, you'll notice they look
almost exactly the same there's nothing
special about each one of these; the only
thing that's different is the number of
neutrons, protons, and electrons each one
has.
Sodium has an atomic number of 11, and
chlorine has an atomic number of 17. At
the moment, this means that it will have
17 neutrons, 17 protons, and 17 electrons,
and since the protons and electrons
balance one another out, the chlorine
atom will also have a charge of zero. The
chlorine atom also currently has an
empty space in its outermost valence
electron shell, just like the sodium atom.
It does not satisfy the octet rule; it's
going to be much easier for a chlorine
atom to gain one electron in order to
fill its outer shell than it would be
for it to lose seven electrons. And so,
that's why you often times see sodium and
chlorine bonded together. An electron
will be transferred over from the sodium
atom to the outermost shell of the
chlorine atom like this. And now both
atoms will have a filled outer shell. the
process doesn't stop here, however, so
let's go back to our sodium atom and
tally up neutrons, protons, and electrons.
If I look at my sodium atom, now I still
have the same number of neutrons and
protons, but we're now missing an
electron. Let me change the number
that's up here like this, so I now only
have 10 electrons, each with a charge of
minus 1, which means that I actually have
more positive charges that i have
negative charges. And so, my final charge
is going to be positive one.
Now go over to the chlorine atom and see
if you can figure out what the charge of
the chlorine atom would be. Remember to
count up the total number of neutrons,
protons, and electrons. If you counted right,
you'll notice that you now have an extra
electron in the chlorine atom. You have
18 negative 1 charges, and you only have
17 positive charges, which means the
chlorine atom would have an overall
charge of minus one after the transfer
of the electron. The sodium atom has a
positive charge, and the chlorine atom
has a negative charge. Because the
charges are opposite to one another,
these two atoms will attract one another,
they'll stick together kind of like
magnets. We call this an
ionic bond.
This molecule of sodium chloride formed
by the ionic bond is actually something
that you're probably familiar with; this
NaCl is actually the chemical formula
for table salt.
Now, let's go back to the periodic table
and see if we can figure out which
columns tend to form ionic bonds. Because
ionic bonds involved transferring a
couple of electrons, we tend to see them
happening in this column over here on
the left, and then we tend to see
them happening in this column that
second from the right over here. So,
elements that are in this column on the
left,
they need to only lose one electron, and
these atoms over here on the right, they
need to gain one electron. Because they
satisfy the octet rule in different ways.
these two element columns tend to
combine with one another pretty
frequently and form ionic bonds.
However, we don't see much bonding action
happening in this column that's farthest
on the right.
Take a look at the number of valence
electrons that each of these elements
has, and see if you can figure out why
these elements are actually called inert
or noble gases. They already have a full
electron outer shell and so they don't
need to form any more bonds in order to
satisfy the octet rule. Because of this,
they don't tend to form bonds. So now we
know that we tend to form ionic bonds
with elements that are on opposite sides
of the periodic table, but what about
these elements that are in here? It would
be a lot of work in order to lose or
gain four or three or even two valence
electrons, and so they wind up doing
something a little bit different. So,
let's take a look at carbon, hydrogen,
oxygen, and nitrogen, just like we're
going to do in class. Let's start with
our friend carbon. Take a look at
its atomic number and see if you can
figure out how many valence electrons
that would have.
If you determined that carbon has two
electrons in its first orbital, and then
another four in the second orbital, then
you're correct. Unfortunately, because it
only has four electrons in the outermost
shell, carbon isn't even close to satisfy
the octet rule, but it would take a huge
amount of effort in order to gain or
lose four electrons in order to satisfy
the octet rule. So, here's what carbon
does instead. Carbon engages in what is
called covalent bonding, where it shares
electrons with other atoms instead of
transferring them. Hydrogen also has one
valence electron, and it needs one more
in order to satisfy its octet rule, and
so frequently you see carbon and hydrogen
bonded together. So, the two of them will
actually share one set of electrons. In
order to fill the outermost shell of
this carbon atom, we're going to need to
add on four hydrogen atoms. Now that we've
bonded together one carbon atom and four
hydrogen atoms, both the hydrogen atoms
and the carbon atoms are satisfied in
terms of the number of valence electrons.
Hydrogen has a full outer shell with two
electrons, because it only has one shell of
electrons in total. And then carbon's
outer shell is also full with eight
electrons. This molecule that we've
created is CH4. also known as methane, or
the gas that you produce when you fart. A
molecule like oxygen has 6 valence
electrons, and so you're going to find
that it will form two bonds with other
atoms. So, you might wind up with an atom
that looks like this and that it looks
like that attaching to this oxygen and
forming a compound that were very
familiar with which is H2O. Both the
hydrogen and the oxygen now have a
complete outermost shell of electrons.
Nitrogen can form three covalent bonds
because it has three spaces to share; one
two, and three, and so a hydrogen might in
theory attach to each of these
attachment points and share an electron
with nitrogen in order to complete that
octet. This compound NH3 is also known as
ammonia, which is a very powerful
cleaning product.
Coming back to our periodic table,
remember that you have ionic compounds
that tend to form with elements on
either extreme of the periodic table, and
you tend to have covalent bonds forming
with elements that are in the middle of
the periodic table. Hydrogen is the
interesting swinger because it can do
both.
However, most of the time in biology, we'll
be discussing covalent bonding because
that's what atoms such as carbon,
hydrogen, oxygen, nitrogen, phosphorus and
sulphur tend to do.
OK, so that's pretty much chemistry in a
nutshell. Thanks for watching, everybody.