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An atom is the smallest part of an element that
still behaves as an element. For example,
an atom of oxygen still behaves like oxygen
even when you've pared it down to just an atom. Atoms come in three parts,
the sub-atomic particles. The first one
is the proton. It has a positive charge
and has a mass. We say that weighs one
atomic unit or amu and an amu is basically
defined as the weight of the proton. On our diagram down here
to the right the proton would be in the center of the atom
as a part of the nucleus. Neutrons are also a part in the nucleus
and they have a charger of 0. They weigh the same amount
as a proton, one
amu. Together
the protons and neutrons in a nucleus give the atom
its atomic mass or atomic weight.
The protons by themselves give the atom
their atomic number. It's the number protons that tells you
how the atoms are going to behave. For example hydrogen has just one proton.
That's why it behaves like hydrogen. That's why it's a gas.
That's why it's highly flammable. Where as oxygen
hasn't an atomic mass of eight, which
gives it its characteristics that make it valuable for us to be breathing.
Now around the outside of the atom
are the electrons. See electrons have a charge
of, a negative charge and their small enough that for the purposes of this
class we're gonna say that they have no mass.
They circle the nucleus and they are what gives the atom
its chemical properties, how it behaves when it gets with
other atoms. In this next section
we're going to talk about bonds and attractions. We're going to talk about
ionic
bonds first, then covalent bonds, of which we have two types: nonpolar or polar.
Then we're gonna talk about hydrogen bonds and hydrophobic interactions.
In this slide we're gonna talk about ionic bonds,
but before we do that let's talk about the arrangement
the electrons around the nucleus. We only have to worry about three orbitals.
They are represented by these lines
around the nucleus. It's kinda like satellites orbiting a planet.
And for this class, we only need to worry about the first three.
Now you'll notice this first orbital is kinda small.
Only two electrons will fit in that first orbital.
In the next two orbitals, eight will fit and
all atoms (if you want to give them human emotions and wants)
want to get up to full
orbitals. It's kinda like playing 21. The closer you can get to 21
then you're closer to winning, you're more likely to win.
So let's take sodium for example. This figure represents sodium. It has
an element number of 11 which means it has
11 protons. It represented by the green
nucleus. Now around the outside, we can get
two electrons to fit in this first orbital, then it's full.
Then we have eight electrons in the second orbital
and it's full. Now we have one left over. We have one electron in this orbital
and that's not really a comfortable situation because
sodium isn't going to go out and get 7 electrons. It's just not going to happen.
Now over here we have chlorine. Chlorine has an element number of seventeen.
That means that it has 17 protons represented
by the yellow nucleus. Now,
in the center we have two electrons in the center, okay, and
that orbital is full. We have eight electrons in the second orbital and
that orbital's full. Then we have seven electrons
in the third orbital. It just needs one more.
Now the more stable configuration would be for sodium
to lose that extra outside electron
and for chlorine to pick up one. So what happens is is chlorine
takes that extra electron from sodium.
Now it has one more electron that has protons.
That means it has a negative charge. Sodium
has one fewer electrons than it has protons. That gives it a positive charge.
Now anytime an atom has
either more or fewer electrons than protons, we call it an ion.
So an ion is an atom the has
a different number of electrons then protons.
Now positive ions are attracted to negative ions.
They hang out together. Those charges attract each other.
Here's another way to think of it, as in a schoolyard
example. Big old mean chlorine is going to beat-up poor
little sodium and has taking his lunch money and
so now chlorine has more money that should
and sodium kinda falls chlorine around hoping that he's going to get his money
back.
Now we have a name for the different types of ions.
A positive ion is called cation
and a negative ion is called an anion.
Let's talk about
atoms that are kinda medium-sized. They're not big enough
to steal from other atoms and they're not small enough to be giving up
electrons to other atoms. Let's take hydrogen for an example.
It actually is quite small, so fits in the "can be stolen from" category.
But it has an atomic number of one.
It has one proton and it has one electron.
Now it would really like to have two electrons
in its outside orbital. So what it does is, it gets together with
another hydrogen and they share the electrons.
See, they go zipping around both protons.
We call this a covalent bond.
And we represent this as a single line
between the letter that designates the element or we can show it this way
with the lower case [subscript] to indicating that there are two hydrogens.
Let's look at when we are combining atoms that are
different. For example, carbon
has an atomic number six so we have a full electron orbital
around the inside and we've got four on the outside.
Now carbon would like to have four more.
It's big enough it's not gonna give up those four electrons
but it really can't take four electrons from another
atom. So in this case, four hydrogens got together.
Remember they wanna share one, and they get together
with a carbon and each hydrogen
is forming a covalent bond with
the carbon and this produces methane,
also known as swamp gas. You'll notice here we're showing a single line
to show were each pair of electrons is being shared.
Let's talk about water. Water, as you may know,
is two hydrogens bonded to an oxygen,
H2O. Oxygen has
an element number of 8. That means it has six electrons in its outside
or valence electron shell. It wants two more electrons.
Hydrogen wants one more. So the two hydrogens get together
with the oxygen and shares with the oxygen.
Now because oxygen is bigger than carbon,
it can keep the electrons a little bit longer
instead of stealing them. And so what happens is that the electron zips around
hydrogen once, twice around oxygen, once around the hydrogen,
twice around oxygen, once around
hydrogen, twice around oxygen, so that what happens is
the molecule gets a slight positive and negative charge.
Oxygen gets a slight negative charge because the electrons are spending more
time around the nucleus of the oxygen
and the hydrogens get a slight positive charge.
We call this a polar covalent bond.
Think about it as the poles of the earth. The earth is a big magnet
and it also has a negative pole and a positive pole.
Now because water has a positive and a negative
side, if you will, a positive and negative pole,
it forms bonds with other water molecules.
The negative oxygen is attracted to the positive
hydrogen and so on and so on. We call these hydrogen bonds.
Don't get confused between the bond, the covalent bond,
the nonpolar covalent bond between two hydrogens in gaseous hydrogen,
and the hydrogen bond between molecules.
Now, hydrogen bonds can form between any molecules
were there is a hydrogen bonded to a oxygen
or hydrogen bonded to a nitrogen. Those are the ones that you need to worry about for this
class.
Now let's talk about what happens when you mix
water with molecules that are not capable of hydrogen bonding.
Here we have an example of a string of carbons,
nonpolar covalent bonds between the carbon
and the hydrogens that are attached to it, and we have a bunch of waters.
Well the waters, because of their polar covalent bonds, they've got this slight
positive and slight
negative charges, they're attracted to each other.
So this one comes over and says, "Hey, I can form a hydrogen bond with you,"
and it's all happy, and this one over here is saying, "oh well don't leave me out
of the party," and
the carbon molecules are saying, "Hey! Hey! You're stepping on my toes,"
and it keeps getting more and more crowded. "Hey! Hey!"
And it can't form hydrogen bonds
with the water because it doesn't have an oxygen
bonded to a hydrogen nor does it have a nitrogen bonded to a hydrogen.
So what happens is, it is excluded by the water.
We call this hydrophobic interactions. Molecules that are
incapable
a forming hydrogen bonds are excluded by the water that wants to hang out by
itself.
Kinda sounds like cliques in high school doesn't it?
Now molecules that can form
interactions hydrogen bonds with water
are called hydrophilic. "Philic" meaning
"loving", "hydro" for "water", so they are water loving molecules, whereas molecules that
are excluded by water that can't
hydrogen bond are called hydrophobic, or water fearing.