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.