SAL: Everything we've been dealing with so far in our
journey through chemistry has revolved around stability of
electrons and where electrons would rather
be in stable shells.
And like all things in life, if you explore the atom a
little further you'll realize that electrons are not the
only stuff that's going on in an atom.
That the nucleus itself has some interactions, or has some
instability, that needs to be relieved in some way.
That's what we'll talk a little bit
about in this video.
And actually the mechanics of it are well out of the scope
of a first-year chemistry course, but it's good to at
least know that it occurs.
And one day when we study the strong nuclear force, and
quantum physics, and all the like, then we can start
talking about exactly why these protons and neutrons,
and their constituent quarks are interacting
the way they do.
But with that said, let's at least think about the
different types of ways that a nucleus can essentially decay.
So let's say I have a bunch of protons.
I'll just draw a couple here.
Some protons there, and I'll draw some neutrons.
And I'll draw them in a neutral-ish color.
Maybe let me see, like a grayish would be good.
So let me just draw some neutrons here.
How many protons do I have?
I have 1, 2, 3, 4, 5, 6, 7, 8.
I'll do 1, 2, 3, 4, 5, 6, 7, 8, 9 neutrons.
And so let's say this is the nucleus of our atom.
And remember-- and this is, you know, in the very first
video I made about the atom-- the nucleus, if you actually
were to draw an actual atom-- and it's actually very hard to
drawn an atom because it has no well-defined boundaries.
The electron really could be, you know, at any given moment,
it could be anywhere.
But if you were to say, OK, where is 90% of the time the
electron is going to be in?
You'd say, that's the radius, or that's the
diameter of our atom.
We learned in that very first video that the nucleus is
almost an infinitesimal portion of the volume of this
sphere where the electron will be 90% of the time.
And the neat takeaway there was that, well, most of
whatever we look at in life is just open free space.
All of this is just open space.
But I just want to repeat that because that little
infinitesimal spot that we talked about before, where
even though it's a very small part of the fraction of the
volume of an atom-- it's actually almost all of its
mass-- that's what I'm zooming out to this point here.
These aren't atoms, these aren't electrons.
We're zoomed into the nucleus.
And so it turns out that sometimes the nucleus is a
little bit unstable, and it wants to get to a more stable
configuration.
We're not going to go into the mechanics of exactly what
defines an unstable nucleus and all that.
But in order to get into a more unstable nucleus,
sometimes it emits what's called an alpha particle, or
this is called alpha decay.
Alpha decay.
And it emits an alpha particle,
which sounds very fancy.
It's just a collection of neutrons and protons.
So an alpha particle is two neutrons and two protons.
So maybe these guys, they just didn't feel like they'd fit in
just right, so they're a collection right here.
And they get emitted.
They leave the nucleus.
So let's just think what happens to an atom when
something like that happens.
So let's just say I have some random element, I'll just call
it element E.
Let's say it has p, protons.
Actually let me do it in the color of my protons.
It has p, protons.
And then it has its atomic mass number, is the number of
protons plus the number of neutrons.
And do the neutrons in gray, right?
So when it experiences alpha decay, what
happens to the element?
Well, its protons are going to decrease by two.
So its protons are going to be p minus 2.
And then its neutrons are also going to decrease by two.
So its mass number's going to decrease by four.
So up here you'll have p minus 2, plus our neutrons minus 2,
so we're going to have minus 4.
So your mass is going to decrease by four, and you're
actually going to turn to a new element.
Remember, your elements were defined by
the number of protons.
So in this alpha decay, when you're losing two neutrons and
two protons, but especially the protons are going to make
you into a different element.
So if we call this element 1, I'm just going to call it,
we're going to be a different element now, element 2.
And if you think about what's generated, we're emitting
something that has two protons,
and it has two neutrons.
So that its mass is going to be the mass of the two protons
and two neutrons.
So what are we emitting?
We're emitting something that has a mass of four.
So if you look at, what is two protons and two neutrons?
I actually don't have the periodic table on my
[? head. ?]
I forgot to cut and paste it before this video.
But it doesn't take you long on the periodic table to find
an element that has two protons, and that's helium.
It actually has an atomic mass of four.
So this is actually a helium nucleus that gets emitted with
alpha decay.
This is actually a helium nucleus.
And because it's a helium nucleus and it has no
electrons to bounce off its two protons, this would be a
helium ion.
So essentially it has no electrons.
It has two protons so it has a plus 2 charge.
So an alpha particle is really just a helium ion, a plus 2
charged helium ion that is spontaneously emitted by a
nucleus just to get to a more stable state.
Now that's one type of decay.
Let's explore the other ones.
So let me draw another nucleus here.
I'll draw some neutrons.
I'll just draw some protons.
So it turns out sometimes that a neutron doesn't feel
comfortable with itself.
It looks at what the protons do on a daily basis and says,
you know what?
For some reason when I look into my heart, I feel like I
really should be a proton.
If I were a proton, the entire nucleus would be a little bit
more stable.
And so what it does is, to become a proton-- remember, a
neutron has neutral charge.
So what it does is, it emits an electron.
And I know you're saying, Sal, you know, that's crazy, I
didn't even know neutrons had electrons in
them, and all of that.
And I agree with you.
It is crazy.
And one day we'll study all of what exists
inside of the nucleus.
But let's just say that it can emit an electron.
So this emits an electron.
And we signify that with its-- roughly its mass is zero.
We know an electron really doesn't have a zero mass, but
we're talking about atomic mass units.
If the proton is one, an electron is 1/1,836 of that.
So we just round it.
We say it has a mass of zero.
Its mass really isn't zero.
And its charge is minus 1.
It's atomic, you can kind of say its atomic
number's minus 1.
So it emits an electron.
And by emitting an electron, instead of being neutral, now
it turns into a proton.
And so this is called beta decay.
And a beta particle is really just that emitted electron.
So let's go back to our little case of an element.
It has some number of protons, and then it has
some number of neutrons.
So you have the protons and the neutrons, then you get
your mass number.
When it experiences beta decay, what happens?
Well, are the protons changed?
Sure, we have one more proton than we had before.
Because our neutron changed into one.
So now our protons are plus 1.
Has our mass number changed?
Well let's see.
The neutrons goes down by one but your
protons go up to by one.
So your mass number will not change.
So it's still going to be p plus N.
so your mass stays the same, unlike the situation with
alpha decay, but your element changes.
Your number of protons changes.
So now, once again, you're dealing with a new element in
beta decay.
Now, let's say we have the other situation.
Let's say we have a situation where one of these protons
looks at the neutrons and says, you know what?
I see how they live.
It's very appealing to me.
I think I would fit in better, and our community of particles
within the nucleus would be happier if
I too were a neutron.
We'd all be in a more stable condition.
So what they do is, that little uncomfortable proton
has some probability of emitting-- and now this is a
new idea to you-- a positron, not a proton.
It emits a positron.
And what's a positron?
It's something that has the exact
same mass as an electron.
So it's 1/1836 of the mass of a proton.
But we just write a zero there because in atomic mass units
it's pretty close to zero.
But it has a positive charge.
And it's a little confusing, because they'll
still write e there.
Whenever I see an e, I think an electron.
But no, they say e because it's kind of like the same
type of particle, but instead of having a negative charge,
it has a positive charge.
This is a positron.
And now we're starting to get kind of exotic with the types
of particles and stuff we're dealing with.
But this does happen.
And if you have a proton that emits this particle, that
pretty much had all of its positive charge going with it,
this proton turns into a neutron.
And that is called positron emission.
Positron emission is usually pretty easy to figure out what
it is, because they call it positron emission.
So if we start with the same E, it has a certain number of
protons, and a certain number of neutrons.
What's the new element going to be?
Well it's going to lose a proton. p minus 1.
And that's going to be turned into a neutron.
So p is going to go down by one.
N is going to go up by one.
So that the mass of the whole atom isn't going to change.
So it's going to be p plus N.
But we're still going to have a different element, right?
When we had beta decay, we increased
the number of protons.
So we went, kind of, to the right in the periodic table or
we increased our, well, you get the idea.
When we do positron emission, we decreased
our number of protons.
And actually I should write that here in
both of these reactions.
So this is the positron emission, and I'm left over
with one positron.
And in our beta decay, I'm left over with one electron.
They're written the exact same way.
You know this is an electron because it's a minus 1 charge.
You know this is a positron because it
has a plus 1 charge.
Now there's one last type of decay that
you should know about.
But it doesn't change the number of protons or neutrons
in a nucleus.
But it just releases a ton of energy, or sometimes, you
know, a high-energy proton.
And that's called gamma decay.
And gamma decay means that these guys just reconfigure
themselves.
Maybe they get a little bit closer.
And by doing that they release energy in the form of a very
high wavelength electromagnetic wave. Which is
essentially a gamma, you could either call it a gamma
particle or gamma ray.
And it's very high energy.
Gamma rays are something you don't want to be around.
They're very likely to maybe kill you.
Everything we did, I've said is a little theoretical.
Let's do some actual problems, and figure out what type of
decay we're dealing with.
So here I have 7-beryllium where seven
is its atomic mass.
And I have it being converted to 7-lithium So
what's going on here?
My beryllium, my nuclear mass is staying the same, but I'm
going from four protons to three protons.
So I'm reducing my number of protons.
My overall mass hasn't changed.
So it's definitely not alpha decay.
Alpha decay was, you know, you're releasing a whole
helium from the nucleus.
So what am I releasing?
I'm kind of releasing one positive charge, or I'm
releasing a positron.
And actually I have this here in this equation.
This is a positron.
So this type of decay of 7-beryllium to 7-lithium is
positron emission.
Fair enough.
Now let's look at the next one.
We have uranium-238 decaying to thorium-234.
And we see that the atomic mass is decreasing by 4, minus
4, and you see that your atomic numbers decrease, or
your protons are decreasing, by 2.
So you must be releasing, essentially, something that
has an atomic mass of four, and a atomic number
of two, or a helium.
So this is alpha decay.
So this right here is an alpha particle.
And this is an example of alpha decay.
Now you're probably saying, hey Sal, wait, something weird
is happening here.
Because if I just go from 92 protons to 90 protons, I still
have my 92 electrons out here.
So wouldn't I now have a minus 2 charge?
And even better, this helium I'm releasing, it doesn't have
any electrons with it.
It's just a helium nucleus.
So doesn't that have a plus 2 charge?
And if you said that, you would be absolutely correct.
But the reality is that right when this decay happens, this
thorium, it has no reason to hold on to those two
electrons, so those two electrons disappear and
thorium becomes neutral again.
And this helium, likewise, it is very quick.
It really wants two electrons to get stable, so it's very
quick to grab two electrons out of wherever it's bumping
into, and so that becomes stable.
So you could write it either way.
Now let's do another one.
So here I have iodine.
Let's see what's happening.
My mass is not changing.
So I must just have protons turning into neutrons or
neutrons turning into protons.
And I see here that I have 53 protons, and
now I have 54 protons.
So a neutron must have turned into a proton.
A neutron must have gone to a proton.
And the way that a neutron goes to a proton is by
releasing an electron.
And we see that in this reaction right here.
An electron has been released.
And so this is beta decay.
This is a beta particle.
And that same logic holds.
You're like, hey wait, I just went from 53 to 54 protons.
Now that I have this extra proton, won't I have a
positive charge here?
Well you would.
But very quickly this might-- probably won't get these exact
electrons, there's so many electrons running around-- but
it'll grab some electrons from some place to get stable, and
then it'll be stable again.
But you're completely right in thinking, hey, wouldn't it be
an ion for some small amount of time?
Now let's do one more.
So we have to 222-radon-- it has atomic number of 86--
going to 218-polonium, with atomic number of 84.
And this actually is an interesting aside.
Polonium is named after Poland, because Marie Curie,
she-- At the time Poland, this was at the turn of the last
century, around the end of the 1800's, Poland didn't exist as
a separate country.
It was split between Prussia, Russia, and Austria.
And they really wanted let people know that, hey, you
know, we think we're one people.
So they discovered that when, you know, radon decayed it
formed this element.
And they named it after their motherland, after Poland.
It's the privileges of discovering new elements.
But anyway, back to the problem.
So what happened?
Our atomic mass went down by four.
Our atomic number went down by two.
Once again, we must have released a helium particle.
A helium nucleus, something that has an atomic mass of
four, and an atomic number of two.
And so there we are.
So this is alpha decay.
We could write this as a helium nucleus.
So it has no electrons.
We could even say immediately that this would have a
negative charge, but then it loses