>> host: This instrument
is called a Geiger counter.
It identifies materials
that are radioactive.
Now watch what happens
as I turn it on,
and move the probe around
some of these items.
Some of them like the soil
or the nuts
that don't have much radiation
may react only a little...
[slow beep]
or not at all.
While this lantern mantle,
you might find
in a camping lantern
or the newer wristwatch
will react some.
And then this older wristwatch
should react a lot-- like that.
[beeping]
What exactly
does "radioactive" mean?
What makes an atom radioactive?
And why is that important
in chemistry?
These are just
a few of the questions
we'll be answering
as we explore
nuclear chemistry
in this unit of
Chemistry Matters!
♪♪
>> host: Welcome to
Chemistry Matters
and Unit 11: Nuclear Chemistry.
When people hear the words
nuclear or radioactive,
they often picture something
out of an old
science fiction movie.
In fact, when teachers
ask students to draw something
they think of when they hear
the word radioactive,
students often draw
destructive explosions,
giant monsters
and wondrous mutations.
But radioactive materials
are a natural part of
the universe,
including planet Earth.
You can find natural
radioactive materials
in the soil, the air
and even inside your body.
In this unit we're exploring
nuclear chemistry,
which is simply the study
of changes in the nucleus
of an atom.
For much of this course,
we've studied chemistry
in terms of what's happening
outside of the nucleus.
When bonds form or break
in chemical reactions,
electrons are moved
or changed.
But the nucleus
stays the same,
so the identity of the atoms
or ions doesn't change.
When water, made of
hydrogen and oxygen,
breaks down into hydrogen gas
and oxygen gas,
the same elements
are involved before
and after
the chemical reaction.
But when uranium
undergoes nuclear decay,
it forms barium
and krypton--
completely new elements!
Elements that can undergo
nuclear changes in this way
are called radioactive.
Uranium also releases energy
when it undergoes nuclear decay.
In fact, that's how
radioactivity was discovered.
In 1896, a chemist named
Henri Becquerel
discovered radioactive decay
in compounds
that contained uranium.
He noticed that containers
of these uranium compounds
made marks
on photographic film.
So we've named the SI unit
of radioactive decay
in his honor.
Scientists call one nuclear
transformation per second,
one Becquerel.
Becquerel discovered
radioactivity,
but it was Marie Curie
who coined the term.
Marie Curie and her husband
Pierre Curie studied
the properties of
radioactive materials,
discovering the elements
Polonium and Radium.
And Marie Curie became
the first person to win
two Nobel Prizes.
And one of only two people
to be awarded Nobel Prizes
in different fields.
The curie, a non-SI unit
of nuclear transformation,
is named in honor
of the Curies.
Just as in all other parts
of chemistry,
when exploring
nuclear chemistry
we maintain a high regard
for safety.
We know that any material
can be harmful
if used carelessly
or in a high dose.
That's why we often wear gloves
goggles and aprons to protect
ourselves from spilled
or splashed chemicals.
We wisely protect ourselves
from anything
that's potentially harmful.
The same is true
when working with
radioactive elements.
We might wear gloves
or aprons,
and we're also careful
to only use low levels
of radioactive materials,
just as we did
when working with acids
and bases.
That way, we can be sure
to avoid any ill effects.
[beeping]
Radioactive materials,
like the ones I have here,
are more common
than you may think.
[beeping rapidly]
The Earth contains
small amounts
of radioactive isotopes,
known as radionuclides,
in our soil, water and air.
They're even found
in our bodies.
Everything from
the smoke detectors
in your home
to the watch on your wrist
can contain
radioactive elements.
While most atoms
are not radioactive,
some atoms have a large number
of neutrons, and that's
one factor that can
make an atom unstable
and likely to decay.
And if you want to find out
whether or not a substance
is radioactive, you can use
a Geiger Counter
like this one
to detect radioactivity.
The clicking or beeping sound
that Geiger Counters make
is caused by
the radioactive decay particles,
seen here in blue.
They enter the counter
and bump into an atom
from the air,
which is depicted in orange.
When the radioactive particle
bumps into an atom,
it knocks off an electron,
which is represented
as the white dot.
That electron hits
the metal rod and causes
a surge of electricity,
which then sets off the alarm
and the meter, in this case,
a click or a beep.
It resets itself,
and that process starts again,
so that we hear the presence
of radioactive materials.
So we can hear and measure
if something is radioactive
by using a Geiger Counter.
But under the right
circumstances
we can actually see
radioactivity, too.
Imagine being able to see
tiny particles leaving an atom
while it's undergoing
nuclear decay.
In this dish, I have some
isopropyl alcohol
on a piece of black paper.
Alcohol is
a volatile substance,
which means it vaporizes
easily-- turning from
a liquid to a gas.
The alcohol is being cooled
by dry ice so that
it doesn't vaporize
as quickly.
You can see tiny droplets.
The droplets look like
a misty cloud inside
a clear container.
And that's where this device
gets its name--
a cloud chamber.
Let's go out to our classroom,
where our students are using
their own cloud chambers
to gain a better understanding
of radioactivity.
>> professor: Let's investigate
what happens to a nucleus
when it undergoes
nuclear decay.
Take a look at the devices
in front of you.
These are cloud chambers.
I want you to use a pipette
to soak the felt at the bottom
of the cloud chamber
with ethyl alcohol
and allow the alcohol
to vaporize.
I've done the same thing
with this larger
cloud chamber here.
It won't take very much
of the ethyl alcohol.
If you hold the cloud chamber
in your hands for a minute,
that will warm the alcohol
and make it vaporize
a little better.
Next, we'll place a small amount
of radioactive material inside.
In this case we'll use,
lead-210 samples
that are mounted on the inside
of a needle.
So place these samples
directly in the hole
in the side of
the cloud chamber.
And don't worry, just because
it's radioactive
doesn't mean it's dangerous.
Next, we'll set
our cloud chambers
on a small insulating pad
and then we'll place the setup
on these blocks of dry ice
and that'll help us visualize
the cold alcohol vapor
inside the chamber.
We'll need to give it
some time to cool down
the alcohol vapor.
Now that
I've lowered the lights,
shine a flashlight
into your cloud chamber
and tell me what you observe.
>> student: Whoa!
Did you see that?
>> student: Yeah,
what are those streaks?
>> student: It kinda
looks like fireworks!
>> professor: Now I want you
to try some of the other
radioactive samples
in the larger cloud chamber.
First the brazil nut,
then a sample of Americium-241,
which is commonly found
in smoke detectors.
>> student: I never thought
that a nut could be radioactive.
>> professor: Well remember,
plants absorb materials
from the soil.
So lots of plants and animals
take in radioactive materials.
>> student: Some of
the vapor trails
have streaks like forks.
>> professor: Well that's
because what you're seeing
are different types of
radioactive particle decay.
In our cloud chambers,
you should have seen something
that looks like this,
and from your remarks
it sounds like you did.
Those streaks that Jaylynn
mentioned are actually
the products of nuclear decay
inside the radioactive atoms
inside the cloud chamber.
The atoms in the material
are emitting particles
from their nuclei.
So what you're actually
observing are the pathways
of those subatomic particles
shooting through
the cloud of alcohol.
>> student: Wait.
I thought you couldn't see
atoms because they were
too small!
>> professor: Good point, Emily!
And you're right.
We're not actually
seeing the atoms.
We're not really even
seeing the subatomic particles
that are being emitted
from the atoms.
But what we can see are
the trails of the particles
that are being left behind
in the condensed
alcohol vapor.
As those particle rush by,
they're pushing the vapor aside,
making it denser in one area
for a brief moment
and that's what produces
those white streaks.
A similar phenomenon occurs
when a airplane
flies through a cloud.
>> student: I saw some streaks
that were bigger
and other streaks
that were smaller.
Were those
all the same particles?
>> professor: Excellent
observation!
You were watching
very carefully.
And if you examine
your evidence,
your observation
is telling us
that there is more than
one type of particle
coming out of an atom.
The larger particles, called
alpha particles
produce the big streaks.
The smaller particles,
called beta particles,
produce much smaller,
fainter streaks
that are harder to spot.
A really interesting project
is to try counting the number
of alpha and beta particles
that are produced per minute
by a variety of materials.
>> student: I never thought
I'd be able to see the path
of a subatomic particle!
That was pretty cool!
>> host: That is pretty cool!
I hope you're able to use
cloud chambers to observe
nuclear decay particles
in your classroom, too.
Our Chemistry Matters
Toolkit has instructions
for making your own
cloud chambers,
small ones like our students
used, as well as bigger
chambers, like this one
we made from a fish tank.
Now before we further explore
nuclear chemistry,
it'll help to review
what we already know
about the structure
of an atom.
In this visual model,
protons are shown in green.
Remember that they have
a positive charge.
In the Net Charge box,
we can keep track of the charge.
When we have one proton,
there's a charge of +1.
Makes sense, right?
We can also see
what element we've built.
With one proton,
or an atomic number of one,
we have hydrogen.
You'll see that
it's labeled as an ion.
Remember that an ion
is a charged atom,
with the number of protons
not equal to the number
of electrons.
Also notice that
the current atom seen here
is stable.
Now let's add an electron
to even up the charge.
Electrons have a negative
charge and are so much
smaller in mass
than protons or neutrons,
that they aren't included
in the Mass Number.
See how the Mass Number
counter stayed at 1,
even though we added
an electron?
Now we have a net charge
of 0, a neutral atom
that's still stable.
Next, let's add a neutron
to our atom.
A neutron has no charge,
so the net charge
doesn't change.
However, a neutron
has approximately the same mass
as a proton, so the mass number
increases to 2.
Remember, the mass number
is calculated by adding
the number of protons
and neutrons together.
As we add protons,
the element changes.
This is because the number
of protons determines
the identity of the atom.
All atoms with two protons
are helium, and all atoms
with three protons are lithium,
and so on.
If we add two more protons
to get to three, with only
one neutron, notice that
the atom begins to shake.
Do you think this is
because the charge isn't zero?
I'll add two electrons
and see if that helps.
Well, we can see that
the charge is now zero,
but the atom is still unstable.
It's still shaking.
What do you think
would make it more stable?
Let's try adding
two more neutrons.
And there you have it.
In this example, adding
two more neutrons causes
the atom to become
stable again.
>> host: So, stability
is determined by the nucleus,
which contains the
protons and neutrons.
As you recall from Unit 3,
atoms of the same element
with different numbers
of neutrons are called
isotopes of that element.
And an element's ratio
of protons to neutrons
plays a big role
in determining whether or not
that element is radioactive.
When an atom is radioactive,
it's so unstable
that it breaks apart,
or decays,
undergoing what we call
fission.
These smaller atoms
are actually new elements
with completely different
properties and structures.
If you need a more in-depth
review of atomic structure,
you can revisit Unit 3.
If not, move on to
the next video on the Unit 11
playlist, where we'll learn
how to write
this fission process
in an equation.