Radiation and Radioactive Decay

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Mr. Andersen explains why radiation occurs and describes the major types of radiation. He also shows how alpha, beta, and gamma radiation affect the nucleus of a radioactive atom. Nuclear equations are also discussed.
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Transcript Provided by YouTube:

00:04
Hi. This is Mr. Andersen and today I’m going to talk about radioactive
00:08
decay or radiation for that matter. What is radiation? Radiation is essentially parts
00:14
of atoms that are given off or energy that’s given off by radioactive atoms. And so we
00:19
measure that using a geiger counter. This is a geiger counter right here. And a geiger
00:23
counter, inside this one you have some inert gases that every time they get hit by a piece
00:27
of radiation they give off a little bit of an electrical charge which can be picked up
00:31
in here. So a regular geiger counter, when you turn it on, you’re going to hear a little
00:35
bit of static. And the reason why is that there’s always background radiation everywhere.
00:40
But is we put something radioactive in front of it. So let’s say we put a big chunk of
00:44
uranium 238 here, it’s going to give off a huge amount radiation and it will be able
00:51
to pick that up. We can use that to do a lot of important things. Like for example date
00:58
how old the earth is. And so there’s a lot of stuff that comes out of that. So how does
01:02
radiation work? Well to understand radiation you have to understand the fundamental forces
01:07
that we have. And so if this up here is a nucleus. So the red ones are going to be the
01:13
protons. The blue ones are going to be the neutrons. What we find is if we look low on
01:17
the periodic table, so this is neon, neon is going to have 10 protons and 10 neutrons.
01:22
But as we move up the periodic table, so here we’ve got the same number of protons and the
01:26
same number of neutrons for calcium. When we move up to tin you’ll find that the number
01:31
of protons is going to decrease compared to the number of neutrons that we have. And if
01:36
we get up to uranium we have 92 protons but 146 neutrons. Well, why is that? Well the
01:42
reason why is that the nucleus is held together using something called the strong nuclear
01:47
force. And so there are all these nucleons held together by this force which holds the
01:53
nucleus together. Now the nucleus would love to shoot apart. And the reason it would love
01:57
to shoot apart is you have all these positive charges up here. All these positive protons.
02:03
And when you have a small enough atom the strong nuclear force is able to hold it together.
02:07
But when you get something like uranium, it’s got 92 protons in the center. And so you have
02:12
to have tons of neutrons to hold it together. And even with those tons of neutrons that
02:17
you have, sometimes it has a tendency to fall apart. And so this chart, it takes a second
02:21
to get your head around it, but essentially what we have on the bottom is the number of
02:25
protons. And then on the side we have the number of neutrons. And so you would think
02:30
if the number of protons and neutrons are always equal, then we have this perfect line
02:34
that goes up here. But as I showed you on the last slide, we tend to get more neutrons
02:40
the farther we go up. And so you can see that this graph tends to drift towards the neutrons.
02:45
So as you get way up here with like 82 protons, the perfect number of neutrons to have would
02:51
be 126. And so what you find is that if an atom exists on either side of this perfect
02:57
line right here, it’s going to give off protons, neutrons. So sometimes they’re going to change
03:04
just to get back to the equal point. Or that perfect ratio of neutrons to protons. And
03:10
so this chart is neat in that it shows you what kind of decay we have. So if you’re on
03:14
this side of this line, you’ll undergo what’s called beta minus decay. If you’re on this
03:19
side, you’re going to undergo what’s called beta plus decay. And then as we get up towards
03:26
here at the end you’re going to have a lot of what’s called alpha decay. And then eventually
03:30
we can have fission as we go far enough up. And so all of these types of radiation are
03:35
ways for an atom to get back to that perfect ratio of neutrons to protons. Okay. So I’ve
03:42
talked about why we have radiation. But what is this radiation? Radiation works like this.
03:48
You can imagine the first experiments where if we take a radioactive material and then
03:53
we allow it to shoot its way through, like this, and then we have a sensor over here.
03:58
If you have something radioactive here and then a tunnel right here, what you’ll get
04:02
is a spot showing up or radiation showing up on this side. So what scientists want to
04:07
do was they wanted to figure out what is the nature of this radiation. So what they did
04:12
is they put a positive charge up here and a negative charge down here. If I remember
04:18
right they actually have negative one one side, but it doesn’t matter. So what they
04:22
found is that you have one spot showing up down here. So there was some particle that
04:26
was going out that didn’t have a charge. We had some that was actually being drawn in
04:31
this direction. And then we had some that was being drawing in the other direction.
04:37
And so we call these things alpha decay. Alpha decay has a positive charge. And so it would
04:44
be drawn towards the negative. So we’re going to have alpha decay down here. Alpha decay
04:49
like that. We’re going to have beta decay up here. Beta decay had a negative charge.
04:53
So it’s drawn towards the positive. And then we have this gamma radiation here that didn’t
04:57
have any kind of a charge. And so what are these types of radiation? First of all if
05:02
we look at alpha decay, alpha decay is simply two protons and two neutrons that are given
05:09
off. And so without electrons what is this? It’s essentially helium. Helium has two protons.
05:16
It has a mass number of 4. And so that’s alpha decay. Alpha decay can’t even move through
05:23
a piece of paper. And so it’s weak as far as that’s going.
05:26
\b \b0 Next type is called beta decay. Beta decay
05:30
is an electron essentially. Electron has a minus one charge. And it has no mass. And
05:37
so that’s beta decay. There’s another type of beta decay, and it’s weird to write an
05:41
electron here, but it has a positive charge. And it also has zero mass. And so we refer
05:46
to that as a positive electron or we call that a positron. But beta decay would be stopped
05:53
by a little sheet of aluminum foil. And then the last type of decay, the one that doesn’t
05:58
have a positive or a negative charge is called gamma radiation. Gamma radiation occurs when
06:03
we have, remember, these strong nuclear forces that are holding this nucleus together. And
06:09
as they start to wiggle and these atoms or the nucleons wiggle underneath their force,
06:16
that energy is given off in the form of gamma radiation. It’s not actually made of nucleons,
06:23
neutrons or protons. But is has a huge amount of energy. It’s like x-rays. And it can only
06:28
be stopped if we move it through large amounts of soil or lead, as a way to stop it. And
06:34
so those are the types of radiation. At least the types of radiation that you should understand.
06:39
And so you also have to write nuclear formulas. And so when we did chemical reactions and
06:46
chemical formulas, remember we had to balance those equations. And you have to do the same
06:49
thing here. And so let’s start with the one that I talked about at the beginning. So let’s
06:53
start with uranium 238. Now uranium 238 is undergoing alpha decay. In other words it’s
07:02
going to lose, let’s go back for just a second. It’s going to lose two protons and two neutrons.
07:09
And so what it’s losing is actually a helium nucleus. And so that has a charge of two,
07:15
two protons and it has a mass number of four. Since there are two protons and two neutrons.
07:21
And so if you think about it, what does this become? Well I’m actually going to start with
07:25
my atomic number and my mass number up here. If you lose two protons, you’re going to have
07:31
a mass or an atomic mass now of 90. And if you lose a mass of four, this is going to
07:38
become 234. Now what’s interesting in a nuclear reaction is we’ve actually changed the number
07:45
of protons. Since you lost two protons it’s not uranium anymore. And so if I look up here,
07:50
here’s uranium on my periodic table. But if I lose two protons from that it’s not uranium
07:56
anymore, it’s going to be thorium. And so this is how you write a nuclear equation.
08:01
You have to make sure that these all balance. The mass numbers balance and the atomic numbers
08:05
balance as well. So what happens to uranium 238. It actually becomes thorium 234 over
08:12
time, as it loses these protons. Okay. Let’s try another one of those. And so let’s say
08:18
this time we’re dealing with beta decay. So let’s say we lose an electron. Electron remember
08:24
has a minus 1 charge. And it doesn’t have a mass number. And so what’s something that
08:28
undergoes beta minus decay, would be cesium 137. So cesium 137 has a mass number of 137
08:37
and an atomic number of 55. And so what happens when it undergoes beta minus decay? Well,
08:44
let’s not write the symbol remember, because that symbol might change. If we’ve got 55
08:49
protons and we lose one electron, what we actually gain is a proton. So this becomes
08:56
56. Where did it come from? One of those neutrons became a proton and gave off this electron.
09:04
So this becomes 56. What is the mass number? Well electrons don’t have a mass, and so that’s
09:09
going to be a mass of 137. Or it’s a negligible mass. And so what does this become? Well we’d
09:15
have to find cesium on here. Cesium is 55. Okay. So here’s cesium right here. Cesium
09:21
is 55 on the periodic table. But since we’re gaining one proton that actually becomes barium.
09:28
And so this is going to become barium 137. Plus that electron which has a minus one charge
09:35
and no mass. And so this would be, we call this beta minus decay. Okay. Let’s do another
09:42
type of decay. And this decay we’ll do is beta plus decay. So what do you lose in beta
09:47
plus decay? You’re losing an electron, but this is a positron. So it actually has a positive
09:53
one charge. It has a mass of nothing. What’s something that undergoes beta plus decay would
09:59
be sodium. Sodium has an atomic number of 11 and a mass number of 22. And so if we lose
10:07
a positron, so if we lose one of these with a mass of this, what do we get? Well, we’re
10:13
going from 11 mass number or an atomic number of 11. We’re losing one of this. This now
10:19
becomes 10. This has not changed at all. So that remains at 22. And so let’s look on our
10:25
periodic table. Well here’s sodium right here. It’s number 11. But if we lose one proton
10:30
we now become neon. And so when you’re doing these equations and writing out the formulas,
10:36
make sure that you go all the way across the top. So we’ve lost one of these. All the way
10:41
across the top here. And make sure that we balance those out. In other words the the
10:46
atomic numbers and the mass numbers of the reactants and the products have to be exactly
10:51
the same. So that’s radioactive decay. And I hope that’s helpful.