Types of Decay

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