A nucleus consisting of a single proton is
very common in the universe and here on earth.
We call it hydrogen.
And as you know, it fuels stars and is the
most common element in your body.
We have oceans full of the stuff.
Next, let's add the other nucleon, the neutron,
and let's start building the universe.
Here we are with hydrogen, a single proton
and a neutron.
Both have an atomic mass number of one, meaning
that a proton and a neutron are close enough
to the same mass that, ordinarily, we don't
have to sweat the fine points.
Now watch what happens when I add a neutron
to light hydrogen.
Voila, presto chango, and the neutron is absorbed
into the light hydrogen nucleus and is changed
into deuterium, or heavy hydrogen.
Chemically heavy hydrogen and light hydrogen
are pretty much the same, both being hydrogen
with a lone electron orbiting the nucleus.
The nuclear properties of deuterium are very
different from light hydrogen, however.
It is this heavy hydrogen that makes heavy
water, which was of such interest during the
second world war for use in the production
of A-bombs and later, after the war, in the
production of H-bombs.
If we add another neutron to deuterium, we
get tritium, yet another form of hydrogen.
However, this is where the fun starts.
Every twelve and a half years, half of the
tritium nuclei will spontaneously convert
themselves to helium three, a rare but naturally
occurring form of helium, and its chemical
properties have changed completely.
Instead of being a form of hydrogen, it has
changed itself into the form of a noble gas.
It has also changed itself in terms of nuclear
reactions.
Tritium is used in H bombs; helium three readily
absorbs neutrons, but that's it, no big bangs.
How is it that tritium can change itself spontaneously
to helium?
Light hydrogen, deuterium, and tritium are
all hydrogen isotopes.
That is, they all have the same number of
protons, in this case one, but differing number
of neutrons, none, one, or two.
Like all isotopes, their chemical properties
are very similar because chemistry is largely
governed by the bonds in between the outer
electrons, but their nuclear properties are
often very different.
It's actually even more interesting than you
think.
The nucleus, if you remember, is very, very
small, and all these positive charges from
the protons are all squeezed together in a
teeny tiny little space.
They don't want to be there, much less somehow
making things more stable as they are squeezed
together.
What in the world is happening here?
The short answer is that we are seeing the
effect of one of the four forces in the universe,
mainly the nuclear strong force.
Let's look at the relative strengths of these
four forces and formally name them.
If we arbitrarily set the strength of the
nuclear strong force to be one, then we find
that the other forces are small indeed.
The electromagnetic, or coulombic, forces
are less than one percent of the nuclear strong
force.
The weak force, which operates only in the
nucleus as does the strong force, is only
one times ten to the minus fifth as strong
as the nuclear strong force.
And finally, poor old gravity is ten to the
minus 39th times as strong as the strong force.
Poor old gravity.
Interestingly enough though, in all modern
theories of cosmology, it is gravity that
finally wins the struggle and overcomes all
the other forces.
The ranges of these forces vary wildly also.
The range of the nuclear strong force is such
that it cannot reach across a medium sized
nucleus.
The coulombic forces reach to infinity.
The weak forces range is 100 times less than
the range of the strong force.
Wow that's small.
And gravity is again infinite in its reach.
So there you go.
All you need to do to make stable nuclei is
just to add lots of neutrons, right?
They add nuclear strong force, but do not
add coulombic repulsion.
Well, for better or worse, it's not that simple.
It's a kind of zen thing; it's all about balance.
Just as with the decay of tritium, too many
neutrons make a nucleus unstable also.
Of all the combinations of protons and neutrons,
there are only a couple hundred that are stable.
A particular combination of protons and neutrons
is called a nuclide.
Let's look at the stable nuclides and some
of the unstable ones also.
Since we are defining things, let's be specific
about terms I have been throwing around.
Z is the atomic number, the number of protons
in a nucleus.
A is the mass number, which is the sum of
z and the number of neutrons.
And in the name U-235, the 235 is the mass
number.
We know that uranium has an atomic number
of ninety-two, because it has ninety-two protons.
So the number of neutrons, by difference,
is 143.
As we have seen, isotopes have the same z,
but differing a's.
U-235 and U- 238 are isotopes.
A nuclide is some specified combination of
protons and neutrons.
An isobar is a group of nucleons that have
the same mass number but differing atomic
numbers.
A radionuclide is a nuclide that is unstable
and decays.
Beta decay is when a neutron is changed into
a proton, and an electron and an anti-neutrino
are ejected from the nucleus.
Beta decay also has an antimatter decay, where
a proton is changed into a neutron, and an
anti-electron, also known as a positron, is
ejected from the nucleus along with the neutrino.
