Hi. It’s Mr. Andersen. And this AP Physics
essentials video 99. It is on the conservation
of nucleon number. Nucleons remember are the
protons and neutrons that we have inside the
nucleus of an atom. And speaking of neutrons,
this is an artist depiction of a neutron star,
which exists after a supernova. And what happens
is there is so much mass it squeezes and many
of those, if not all of the protons, are converted
into neutrons. And so you are left with a
star that has twice the mass of our sun but
it is going to have a diameter of around 7
miles across. Incredibly density. So conservation
of nucleon number and charge will always exist
in every nuclear reaction and also in radioactive
decay, or nuclear decay. Remember nucleon
number is going to be the number of protons
and neutrons. And then charge is going to
be the number of positive and negative charges.
Before have to equal after. And so a nuclear
reaction the two types we will talk about
are fission and fusion. In fission remember
we are taking a nucleus and we are breaking
it in half or we are splitting a nucleus.
In fusion we are fusing two nuclei together
to make a new nucleus. Nucleon number before
and charge before has to equal nucleon number
and charge after. In nuclear decay, that is
when an atom is giving off radiation, it is
releasing a certain amount of energy. It comes
in three forms or many forms, alpha, beta,
gamma are the ones we will talk about in this.
And in radioactive decay nucleon number and
charge before and after are going to be equal.
Before we get to that, you should understand
A Z X notation which is a way to write down
the number of protons and indirectly the number
of neutrons inside any particle. And so the
X will be representing the chemical symbol,
but it could represent a form of radiation
like an alpha particle. Z is going to represent
the atomic number or the number of protons.
That is going to be the numbers we see on
the periodic table. And then A represents
the mass number. So it is the number of protons
plus N which is going to be the number of
neutrons. And so if we are looking at uranium
235, you know immediately that this in uranium.
The symbol tells you that. You know that it
has 92 protons. And then we know the number
of nucleons is 235. So I could solve indirectly
that the number of neutrons is going to be
143. And so remember we are in physics not
chemistry. In chemistry the electrons are
important. So for combusting some methane
in the presence of oxygen, it is the electrons
that are interacting and it is the atoms that
are conserved. But now we are just dealing
with the nucleus itself. And so it is not
about the electrons anymore, it is about the
nucleons themselves. And so let’s start
with fission, which is how we take a nucleus
and we split it in half. Let’s say we hit
uranium 235 with a neutron. It quickly breaks
apart into krypton 92 and barium 141. We also
get 3 new neutrons as well. And so let’s
study conservation of nucleon number and charge.
The best way to do that is to write out our
nuclear reaction. So I have my 1 neutron before
and uranium before. Then I have my barium,
krypton and my 3 neutrons after. We also have
to see the charge. And we have to see the
nucleon number. So we will write those all
out in AZX notation. And so let’s start
with nucleon number. How many nucleons did
we have before the reaction? Well we had 235
in the uranium plus the 1 neutron so we had
236 to begin with. How many do we have when
we are done? 141 plus 92. So that is going
to be 233. Plus 1, 2, 3 neutrons. So it is
conserved. The nucleon number before and after
is exactly the same. We could also look at
the charge. So what is the charge of uranium?
92. A neutron has no charge remember. And
so 92 has to equal the charge of the barium
and the krypton together. It is those protons
in those two nuclei. If we were to look at
fusion, fusion we have 2 nuclei that fuse
together. An example we would see in the sun
is due to deuterium and tritium. And so those
are two isotopes of hydrogen. And so it is
hydrogen but it has a neutron. We sometimes
call it a heavy hydrogen. And so if we were
to take one that has an extra neutron and
one that has 2, those can be fused together
in the sun and we form a new nuclei, we form
a new atom. We form helium. And so we give
off one neutron and we give off a huge amount
of energy. That is the energy you are utilizing
right now. So let’s make sure that we have
conservation. We first of all write out what
are the particles before and after. What are
the nuclei before. We had the hydrogens before
and then we have the helium and the neutron
when we are done. We now right it in AZX notations.
So it would be like this. This is the deuterium.
So it has 1 proton 1 neutron. And then this
is the tritium, 1 proton, 2 neutrons. And
so is nucleon number conserved? Well, when
we are done we have 5 nucleons to begin with.
We have 5 nucleons when we are done, so it
is conserved. If we look at the charge, we
had 2 positive charges before, 2 positive
charges after. And so charge is going to be
conserved as well. Now radioactive decay is
how an atom gives off energy through radiation.
It comes in many different forms. The ones
we will talk about are alpha, beta and gamma.
They have certain amounts of energy. You could
stop an alpha particle with a little bit of
paper. Beta requires aluminum. But gamma has
huge amounts of radiation and requires a long
section of lead to actually stop the gamma
radiation. But if we look at alpha decay,
alpha decay you are giving off an alpha particle.
An alpha particle is two protons and 2 neutrons.
So we do not have the electrons. So if we
were to look at decay of uranium 238, what
I have done is I have written uranium 238
over here. This is our alpha particle. But
what we are going to try to do is, using the
conservation of nucleon number and charge,
figure out what we are left with after that
decay. Well let’s start with a mass number.
So we had 238 before. We have a mass number
4 nucleons after. So what has to be here?
That is going to be 234. So it is conserved.
If we look at charge we had a positive 92
to begin with. We lose 2 positive charges
in the alpha particle. And so what is going
to be right here? It is going to be 90. Once
we know that, its atomic number, then we can
figure out that this is going to be thorium.
So you can work backwards to figure out what
is produced. If we look at one form of beta
decay, beta decay is kind of crazy. It is
when we take one neutron. We convert that
into a proton. We also give off an electron
and then we make an electron antineutrino.
And so if we were to look at beta decay, for
example of carbon 14, carbon 14 you can see
the number of charge and the nucleon number
up here. So we are giving off an electron.
So it has a negative charge, electron antineutrino
which does not have any mass or charge. So
we can pretty much ignore that here. So now
let’s work backwards. What is going to be
our nucleon number here? Well we did not lose
a proton. We did not lose a neutron. We just
converted one into the other. And so we know
that that is going to stay at 14. We could
look at the charge. We had a positive 6 to
begin with. We get a negative charge here.
And so that means that this cannot be 6 anymore,
that has to be 7. So we have conservation
of electric charge. Since we have changed
that to 7 protons now we have a new atom.
And so this is going to be nitrogen. In this
form it would be beta minus decay. And also
in gamma decay. So if we were looking at gamma
radiation, that is high energy electromagnetic
rays that are given off. And so this would
be a gamma particle right here. It has no
charge and it has no mass number. So let’s
work backwards. What is going to be the nucleon
number? If we started with 137 we have to
have 137. If we look at the charge, since
we have not lost any charge, we are going
to have the same charge and the same number
of protons. And therefore we have barium again.
And so you may think to yourself, well, what
happened? Where did that energy come from?
And so lots of times we will denote that with
an asterisk which means this is excited barium.
That means there is energy in the position
of the electrons themselves. And so did you
learn to apply the conservation of nucleon
number and electric charge to all forms of
radioactive decay and nuclear reactions? I
hope so. And I hope that was helpful.
