### Video Transcript

In this video, our topic is nuclear
fusion. This is a process that takes place
all throughout the universe, in the core of active stars. Nuclear fusion is behind all the
light and heat that the Earth receives from the Sun in our solar system. From this fact, we get a sense for
just how much energy is available through this phenomenon.

Talking about this topic, let’s
start out with the definition. Nuclear fusion is the joining of
more than one atomic nucleus to create a single nucleus. This basic idea then is that if we
have two separate atomic nuclei and they come together and form a third combination
nucleus, then that’s fusion. Let’s look at an example of
this.

Say that we have a hydrogen atom
nucleus. That’s right here, where the blue
dot represents a proton and the green dot represents a neutron. So this is our nucleus. And if we were to write this down
as an atomic isotope using symbols, then we would write it as capital H, since this
is hydrogen which has an atomic number of one. And it has a mass number of two,
since there are a total of one plus one protons and neutrons. So that’s our first nucleus. And say that the second nucleus
that will come together with this one to fuse is identical to it. It’s also hydrogen two. In other words, it’s a hydrogen
nucleus that has one neutron in it. So its mass number is again
two.

Now let’s say it happens that these
two hydrogen nuclei collide with one another. This is actually harder than it may
at first seem. After all, these two nuclei both
have a net positive charge. So they’ll resist being pushed
together. But if we’re able to overcome that
repulsion and actually get the two nuclei to collide and fuse, then here’s what can
happen. The two hydrogen nuclei come
together, join up, and create a third fused nucleus, now with two protons and one
neutron in it. And then along with this fused
nucleus, there’s a free neutron that’s released.

Considering this fused nucleus,
since it has two protons in it, that must mean that it has an atomic number of
two. And as we look that value up on the
periodic table, we see that it corresponds to helium. So these two hydrogen nuclei have
come together to form a totally new element, helium. And then in addition to that fused
nucleus, there’s a neutron that’s released.

Now that we have this fusion
reaction, let’s consider the atomic numbers as well as the mass numbers on either
side of it. We can see that the atomic number
of each one of these hydrogen nuclei is one. So if we total them together, we
get one plus one, two. Then looking on the other side of
the equation, we have the two protons in the helium nucleus and then no protons in
the neutron. This tells us that, in this
reaction, atomic number is conserved from the beginning to the end.

Now what about the mass number, the
number of protons plus neutrons in each of these constituents? The two hydrogen nuclei both have
mass numbers of two. So that gives us a total of two
plus two, or four. And then on the right-hand side,
the helium nucleus has a mass number of three. And if we add that to the mass
number of the neutron, we once again get four. So mass number as well as atomic
number is conserved across this reaction. Okay, so that’s true. But here’s where things get
interesting.

If we were able to measure the
total mass, not mass number, but mass on either side of this equation — say we were
to put the two constituents on either side onto a scale — then we would find that
the total mass of the two hydrogen nuclei before the fusion occurred is greater than
the mass of the products of that fusion, the helium nucleus and the neutron. But wait! How could that be? Because we just counted up the
number of protons and neutrons and found they’re consistent on either side.

Well, it turns out that some of the
mass in an atomic nucleus, like the nuclei of our two hydrogen atoms or the nuclei
of this helium atom, is used up — we could say — as a glue that holds the nucleus
together. For example, say that you had a
whole lot of small wooden balls and you wanted to find a way to keep them all
attached together. One great way to do that would be
to glue all these wooden balls together. Now of course the glue itself has
some mass. So if we were to calculate the
total mass of this collection of wooden balls, we would include the mass of the glue
along with the balls.

It’s a similar idea over here with
our hydrogen nuclei. When they fuse together into one
combined nucleus, the helium nucleus, some of the glue — we could call it — that
kept these two hydrogen nuclei together is not needed to glue together this
resulting helium nucleus. To make this fused helium nucleus,
we need less glue than we needed for the two hydrogen nuclei. And that’s why if we were to weigh
out the reactants of this process against the products of the process, we would find
the total reactant mass is greater.

By the way, the technical term for
this glue that holds nuclei together is binding energy. So interestingly, just for
nucleons, protons, and neutrons in order to be able to stick together, that takes a
little bit of energy in and of itself. When fusion takes place, the
binding energy that previously went into holding these two hydrogen nuclei together
that isn’t needed to hold the helium nucleus together is released.

Just to show that, we could add on
an energy term on the product side of this reaction. This energy is the binding energy
that’s no longer needed to fuse together this resulting nucleus, in this case our
helium three nucleus. It’s because of this energy
released that the process of fusion is so useful at generating energy. Fusion is the core process that
takes place in our Sun. It’s the reason behind all the
light and heat we receive from the Sun.

Now at this point, it’s worth
saying a word about what nuclear fusion is not, because there’s actually a nuclear
process which sounds similar but is quite the opposite. As we’ve seen, nuclear fusion
involves the joining of more than one atomic nucleus to create a single resulting
one. This is in contrast to the process
known as nuclear fission, which involves the splitting of a single nucleus into
multiple smaller ones. So if we take a large atomic
nucleus and break it up into smaller pieces, that’s fission. But if we take small atomic nuclei
and fuse or join them together to make a larger one, that’s fusion.

Since both these processes involve
atomic nuclei and they’re both used to generate energy, it can be confusing to keep
the two separate. One way to do this is to realize
that the word “fusion” means to fuse together or join separate parts and that this
is the opposite of fission, which involves splitting apart.

Now if we look up this process of
nuclear fusion online, one of the things we’ll find is that, even though this
process occurs regularly in the cores of stars, finding a way to bring the process
down to Earth, so to speak, has been quite a technological challenge. Say that we wanted to build a
facility where we could have nuclear fusion going on for the purpose of generating
power. For a few reasons, this seems like
a really great idea. First, this process obviously
works. Consider all the energy created by
our Sun, for example. And also the ingredients — we could
call them — the elements involved in this process, hydrogen and helium, are very
common on Earth. This means that it shouldn’t be
hard to find fuel for a fusion reaction. And it also means that the products
of that reaction will be easy to work with. They won’t be dangerous or
radioactive or need very special handling.

All in all, there are a lot of
great advantages to the process of nuclear fusion as an energy supply source. But to get a sense for the
challenges involved in making this process work on Earth, consider where fusion
happens now. It happens in the core of stars,
specifically where temperatures are in the tens of millions of degrees Celsius. This high-temperature, high-energy
environment is no accident.

Remember, we said that, in order
for fusion to occur, say for our two hydrogen nuclei to come together and to fuse
into one nucleus, it’s necessary to overcome their mutual repulsion, since after all
these two nucleus have an overall positive charge and therefore push one another
apart. From that perspective, we could say
that fusion doesn’t want to happen. Electrically, these nuclei want to
repel each other. In order to make fusion happen, we
need to put so much energy in the environment of these nuclei that that energy is
able to overcome this repulsion. And that’s why fusion only happens
in places where the temperature and therefore the energy is very high.

So in order to make fusion work on
Earth, we need to somehow create an environment that’s able to handle temperatures
in the tens of millions of degrees. Various ideas for how to do that
exist. And it’s an ongoing process. We’re still figuring it out. For our purposes though, we want to
focus on what fusion is and how it works. To better understand that, let’s
consider this example.

Say that we, once again, have two
hydrogen nuclei. And as before, the blue dots
represent protons and the green dots represent neutrons. In our earlier example, both our
hydrogen nuclei had one neutron. But now one of them has a single
neutron and the other has two of them. If we write out the symbols
representing these hydrogen isotopes, one would be hydrogen two and the other would
be hydrogen three.

Now before we go further with this
fusion reaction, it’s helpful to realize that these particular isotopes of hydrogen
come with special names. We can call them according to their
mass number, hydrogen two and hydrogen three, respectively. But it turns out that these
particular isotopes of this particular element have the names deuterium and tritium,
respectively. There’s nothing wrong with calling
them hydrogen two and hydrogen three instead. But if you come across these names,
just know that they refer to the same things. And a helpful way to remember which
name goes with which isotope is to know that tritium has this prefix “tri” meaning
three and deuterium has the prefix “deu” meaning two. So anyway, those are names for
these hydrogen isotopes we may sometimes encounter.

So let’s say we take these two
nuclei and we fuse them together. In other words, we put them in an
environment where, instead of repelling one another, they actually join to create a
new combined nucleus. Now in this reaction, one free
neutron is released, like we saw in the previous fusion reaction. But in addition to that, there’s
also the main fused nucleus that results as a product. The question in this example is,
“What is that main fused nucleus? How do we represent it as a
symbol?”

To figure this out, to see what
atomic isotope is formed in this fusion reaction, we can use the fact that atomic
number is conserved on either side of the reaction, as is mass number. In other words, the total atomic
number on the left side of the reaction equals the total atomic number on the right
side, and the same thing for the mass number. This is an equivalence between the
product side and the reactant side of a nuclear reaction that we can generally
assume.

So if we start with atomic number,
on the left-hand side of the reaction, we have a total atomic number of one plus
one, two. Now on the product side, our
neutron has an atomic number of zero, which means that, whatever our fused nucleus
is, it must have an atomic number of two. That’s to make the total atomic
number on this side of the equation agree with the total on the other.

Now if we look up on the periodic
table of elements what element is number two, that is, has two protons in its
nucleus, we see that the answer is helium, symbolized He. So our fused product nucleus is a
helium atom. And we now just wanna figure out
how many neutrons are in the nucleus of that atom. To solve for that, we’ll balance
the mass number on either side of this equation. On the left-hand side, our total
mass number is two plus three, or five. And on the right-hand side, our
total mass number is one plus the mass number of this helium atom. The number we need to add to one in
order to raise it to five is four. Therefore, that’s the mass number
of this helium nucleus.

So we’ve answered the question of
what atomic element and what isotope of that element is formed in this fusion
reaction. Like the reaction we saw earlier,
we took hydrogen and fused hydrogen nuclei together to create helium plus a free
neutron. This reaction form, adding hydrogen
to hydrogen to create helium, is very common in fusion processes. The reason for this is that, by
doing the fusion reaction this way, combining hydrogen to make helium, we get the
largest energy yield from the fusion that goes on. So whenever we see a nuclear
reaction where hydrogen is used to create helium, it’s a good guess that this is a
fusion process we’re seeing. To get just a bit more practice
with these ideas, let’s try another example.

The following nuclear equation
shows two hydrogen nuclei fusing to form a helium nucleus. What is the value of 𝑚 in this
equation? What is the value of 𝑛 in this
equation?

Taking a look at the nuclear
equation, we see these two hydrogen nuclei, which are fusing, we’re told, to create
helium plus the release of energy. We also see that the atomic numbers
as well as the mass numbers of these hydrogen nuclei are shown, whereas the atomic
number of helium and its mass number are not shown. It’s those values we want to solve
for. And we’ll do it by using the fact
that atomic number and mass number is conserved in this reaction. That means that if we add together
all the atomic numbers on the left side of the equation, that sum will equal the sum
of the atomic numbers on the right side, and same thing with mass number.

Summing the values on the left side
will equal the sum of the values on the right. Now on the right-hand side, since
our only products are a helium nucleus plus energy, we know that only the helium
nucleus will contribute in terms of mass number and atomic number. The energy that’s released in this
fusion reaction has no charge and it has no mass. This means that when it comes to
answering our first question, what is the value of 𝑚, the mass number of helium, we
can write that the sum of the mass numbers on the left-hand side of our equation one
plus two is equal to 𝑚, the mass number of the helium atom. And this tells us that 𝑚, the mass
number of that atom, is three.

Then moving on to solve for the
value of 𝑛, the atomic number of helium, there are a couple of ways we could do
this. One is to look up helium on the
periodic table of elements and see what element number it is. Another way to solve for 𝑛 is to
realize that it must equal the sum of the total atomic numbers on the left-hand side
or the reactant side of this equation. So the atomic number of our first
hydrogen atom plus the atomic number of our second is equal to 𝑛, the atomic number
of helium. And we find that 𝑛 is equal to
two, a result we could find using either one of these two methods, either using the
periodic table or the fact that atomic number is conserved in this equation.

Let’s summarize now what we’ve
learned in this lesson about nuclear fusion. We’ve seen that nuclear fusion is
the joining of more than one atomic nucleus together to create a single resulting
nucleus. This is the opposite, we noted, of
the similarly named process of nuclear fission, where atomic nuclei are split
apart. The energy generated by fusion
comes from what’s called the binding energy that exists between protons and neutrons
in an atomic nucleus. As we studied fusion reactions, we
learned that the most common form is where hydrogen nuclei fuse together — they join
— to create helium. Along with all this, we saw that
while nuclear fusion is an ongoing constant process in the core of our Sun, we
haven’t yet found a practical way of reproducing this process on Earth in a nuclear
reactor. Nonetheless, nuclear fusion shows
great promise as an energy supply source.