Hi. It's Mr. Andersen and this AP Physics essentials video 3. It is on the Atomic Nucleus
which was discovered by this man, Ernest Rutherford. JJ Thompson had already discovered the electron
which they knew had a negative charge. But they had viewed the atom as a plum pudding.
And this is what plum pudding looks like. It is got little plums in it. And then it
has got the pudding portion. So they thought the atom looked like this. It was this amorphous
positive charge inside the atom and then these little negative charges were interspersed.
And so what Rutherford did is he shot alpha particles at it. Those have a positive charge.
So he assumed they would just go right through since there was no positive centering of that
positive charge, that they would just kind of move through in a straight line. But what
he found is that these positive charges went straight on through but occasionally would
come shooting right back at him. It would be bent in some form. And so he said it was
like shooting a shell at a kleenex and that shell just came shooting back at him. And
so what he discovered was this really dense tightly packed positive nucleus which we know
now is made up of positive protons but also these neutral neutrons. Now the protons are
important because the number of protons tell us what the element is. In other words if
it has six protons we know that it is going to be carbon. If it has one proton then we
know it is going to be hydrogen. Now you can have something that is the same element, but
varying amounts of neutrons. And so if we add up the number of protons and neutrons
and they are different in a given element we call those isotopes of that element. And
so for example carbon 12 is going to have 6 protons. So is carbon 14 because they are
both carbon. But carbon 12 is going to have 6 additional neutrons and carbon 14 is going
to have 8 additional neutrons. And so we are going to have these different isotopes. And
we will find that for all of the different elements. Some of these are radioactive. And
what that means is they are unstable and they have a potential to decay or to fall apart.
They give off radiation when they do that. And the rate at which they do that is known
has their half-life. And so again the atomic nucleus is made up of two subatomic particles.
We call those protons, which have a positive charge and neutrons which have no charge.
You could count the number of positive charges right here and figure out what the element
is. If we organize the protons in a certain fashion we get the periodic table. And so
we know that hydrogen has 1 proton. That is what the atomic number means. We know that
iron has 26. We know that gold has 79. It tells us what the element is. But you can
have atoms of the same element and varying numbers of neutrons. And when we do that we
create something called an isotope. And so if we look low on the periodic table we find
uranium 92. That means it has 92 protons. But there are going to be three naturally
occurring isotopes on our planet. Uranium 238 is going to have 146 neutrons. Uranium
235 will have 143. And Uranium 234 will have 142. Where did I come up with those numbers?
If I take 238, which is the sum of neutrons and protons and I subtract 92 that tells me
the number of neutrons. And so on our planet we are going to have varying amounts of that.
And the average of that gives us the average atomic mass. Now if we look at this graph
right here, this is graphing the number of protons along the x-axis and the number of
neutrons along the y-axis. And so this would be that virtual straight line, if the number
of protons and the number of neutrons are equal. And you will find right here the average
which is this jaggy line right here starts to drift towards the neutron side. What does
that mean? As our atoms get larger and larger and larger and as our atomic nucleus gets
larger and larger and larger, you have to have more neutrons to maintain the stability
of the atomic nucleus. And we will talk about that in later videos. But what you are create
and what the colors on this graph are are different types of decay or isotopes breaking
down or they are giving off what is called radiation. And so radioactive decay is when
the atomic nuclei breaks down. And we could summarize that in three different types of
radiation. You have an alpha particle and that is two protons and two neutrons that
are given off. You could get a beta particle. And that is either going to be an electron
or a positron. And then you have this high energy high level gamma radiation. It is electromagnetic
radiation. So this would be an alpha particle given off by a nucleus. And so it becomes
a different element. And so if we talk about an example of that, uranium 238 naturally
occurs on our planet. But it is going to undergo decay. It will lose an alpha particle. And
so as it loses that it is losing four of these nucleons, these parts inside the nucleus.
And so you can see that the mass number has changed. But it has also become a new element.
Since you have lost two of these protons it is not uranium anymore it is thorium. It could
undergo then beta decay so we lose a beta particle. And it becomes protactinium 234.
It could lose another beta particle and it could become uranium 234. And so each of these
have a probability of occurring and that probability is going to be in the atom itself. And so
if we take a sample of 238, we can create a curve of what is called its half-life, which
is the ability (or time) for half of the atoms in that sample to decay or to breakdown. And
so if we look at uranium 238 at the beginning of time, so time is graphed along the x-axis.
At time 0 we are going to have 100 percent of that uranium 238. In one half-life, that
is what this 1 on the x-axis stands for we are going to have 50 percent of that uranium
238 decay. We are going to lose those alpha particles. Now what is the half-life of uranium
238? It is a ridiculously large number. It is 4.5 billion years which is about the same
age as our planet. And so we would expect in that first 4.5 billion years we are going
to have half of that uranium 238 decay. And then the next 4.5 billion years we will go
from 50 percent to 25 percent. And then to 12.5. And it will keep following that at a
known rate because there is a known probability of each of these atoms decaying during that
period of time. Now what is cool about this is scientists can find a sample of uranium
238. We could figure out how much of it has decayed. And we could find it along this line
and that would tell us how long ago that uranium 238, let's say a rock for example, had actually
formed. And so did you learn to describe how the internal structure of a nucleus or an
atomic nucleus relates to the properties of that atom? Remember the protons tell us what
it is and if we add the protons and neutrons we get the isotopes which can be stable or
unstable and can breakdown over time. And I hope that was helpful.