The Cosmic Classroom - Fusion, Fission, and Why Not Iron?


Uploaded by vmargoniner on 30.10.2009

Transcript:
Hi there welcome back to the cosmic classroom.
Today we are going to talk about fusion fission
and why is it that stars can not fuse any element
heavier than Iron. So let's start by
discuss, discussing these two processes.
To fuse something is to bring two things together
and create something that has more mass.
As if you were to take two pieces of a LEGO
combine it together and create one that it's bigger.
Fission is the opposite process, is the opposite,
the process of breaking those things apart, of
getting one big thing and breaking into more
than, than one element. So for example one
fusion reaction that happens in the interior
of stars is that three atoms of Helium combine
to form one atom of Carbon. So it forms one atom of
Carbon, and what's, nice and what helps the star is
the fact that it releases energy in this process.
Alright? At the end you have the
same number of Protons and the same number of
Neutrons that you started with. But somehow some release,
some energy was released. So you probably heard that,
oh yes, when you go from three atoms of Helium to
one atom of, Carbon you lose some mass and
energy is generated would, using with the process that
it's explained by Einstein's famous equation E=MC².
But lets think about this with a little bit more
depth in, in here. I'll get to it in a minute
but now I see ok, so this is fusion, this is
fission would be exactly the reverse.
Right? It's to break this Carbon.
Notice that to break an atom of Carbon you
need to add energy to it. Alright?
So can you do that? Yes you can, but you
need to have the energy available to
give to Carbon to be able to fuse it into
three atoms of Helium. So let's think about the
structure often, of an atom again, it's pretty, it's
pretty simple to think about the nucleus of an atom.
You have two particles, Proton and Neutron, and
thenthe number of Protons defines the element.
And usually you have the same number of
Neutrons and Protons. So if you look at this
next slide an atom of Hydrogen has
one Proton,an atom of Helium has
two Protons and two Neutrons, an atom of
Carbon has six of each one, and an atom
Iron has twenty-six Protons and twenty-six Neutrons.
Now,if you were to try to move, if you
were to measure the mass of each one of
those atoms, obviously the mass of an atom
of Carbon would be greater than the mass
of an atom of Helium. You can see that
Carbon is much bigger has more particles it
would be more massive. But the funny thing is
that if you weight this atom of Carbon and
you weight the atom of Helium, the atom
of Carbon is less than three times heavier
than those three atoms of Helium that it came from.
You have the same number of particles.
Right? You had, you had
three times four particles in Helium
and now you have one times
six particles in Carbon 12 and 12.
So why are we losing mass?
Why is it that we weigh Carbon, and it weighs
less than three atoms of Helium in the other side?
To answer that question you need to start thinking
about what is mass. Mass you know
our everyday life, we just think what's
the amount of stuff. But what mass really
is, in its more, more basic nature, is
how hard it is to make something move.
So something that has a lot of
mass is harder to move than something
that has very little mass. So what I'm saying is, it's
easier to move one atom of Carbon than it is to
move three atoms of Helium. Right?
So if you were to weigh, to measure the mass of
each one of those atoms you would find that the
atomic mass of the Hydrogen is 1.0079
some units, we don't need to go there now.
But some units of mass. Helium too has 4.0026 and
so forth, you can see that. Now let's divide this
atomic mass by the number of nuclei in
each one of those atoms and compute
the mass of each nucleus, the mass of
each Proton in there. If we do that we'll notice
that the mass of, of the Proton that belongs to
Hydrogen is greater than the mass of a Proton
that belongs to Helium. Which is greater than
the mass of a Proton that belongs to Carbon?
Which is greater than the mass of a Proton
that belongs to Iron? Ahh?
So one,what I'm saying is that 1.0078 is greater
than this number,which is greater than this,
which is greater than that. They have the same
number of particles but the one way to
think about this is that the, maybe the
Protons that belong to an atom of Hydrogen
are kind of chubby. You know,they are pretty big.
And as you add more and more particles
the atoms get thinner and thinner and thinner,
until an atom of Iron. Iron is this, Iron
has particles that are as small as they can get.
That has the smallest mass of all of the other elements.
It's easier to push one Proton of Iron than it
is to push one Proton in an atom of Hydrogen.
Alright? And it's this difference
in mass that results in the energy that we see
released by the star or by whatever
elements that's,that's been fused together.
So notice that no atom, and I didn't show them
out to you here you are going to need to trust
me, that no atom in the whole periodic table
has less mass per nucleus than Iron.
And the thinner that is, that's why for you
to make any other element you need to
give it some energy, give it some food, for
it to fatten up and be able to participate in
another kind of element. Yeah?
So you can do fusion with Iron and
it is done when there is a supernova explosion
and it is the energy around,but it takes the
energy, it takes feeding this atom and make it
takes feeding this particle and making
them all a little bit fatter. Alright?
So it doesn't help a star. So doesn't create energy.
So it's not good for the star and that's why it
doesn't happen in the interior of the star, because
if it did, it would just make the star die even faster.
It would take energy from the star to make
something heavier. Now, if you try to do this
at home, be aware of one thing it took me an hour this
morning to figure this out. If you look at the
periodic table there you'll see that most
of them will say atomic mass, and the
atomic mass that they have listed it's not
really an atomic mass it's an atomic weight.
Which is the average of the masses of all of
the Isotopes out there. Right?
Isotopes with more or with less Protons.
What you need to know is the atomic mass of each
isotope that's being created. So you need to make
sure that the atomic table, that the
periodic table you're looking at has atomic
mass not atomic weight. The way to figure it
out to know if you're looking at the right
table, look at Carbon. Carbon has an atomic
mass of 12 because that's how it was defined.
The mass of one Proton is the mass of the
nucleus of Carbon divided by 12, that's the definition.
So look at a table. If it's one you're looking at the right
thing,if not you're not. So I hope you understand
now that the reason why we can not fuse high,
iron, is because we need to give energy to iron
and in huge amounts to, to, to make them, them
each of their nucleus bigger. Alright?
So you can get release by fusing
elements that are lighter than iron and building
blocks that are bigger. Or you can start from
elements that are much heavier than iron, split
them with fission and that will also release energy.
It's okay to do the reverse, but you need to give
energy to the process instead of getting
processed, getting energy. I hope this helps your
understanding of fission fusion and what's so
special about Iron.