WEBVTT

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SAL: Everything we've been
dealing with so far in our

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journey through chemistry has
revolved around stability of

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electrons and where electrons
would rather

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be in stable shells.

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And like all things in life,
if you explore the atom a

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little further you'll realize
that electrons are not the

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only stuff that's going
on in an atom.

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That the nucleus itself has some
interactions, or has some

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instability, that needs to
be relieved in some way.

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That's what we'll talk
a little bit

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about in this video.

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And actually the mechanics of
it are well out of the scope

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of a first-year chemistry
course, but it's good to at

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least know that it occurs.

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And one day when we study the
strong nuclear force, and

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quantum physics, and all the
like, then we can start

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talking about exactly why these
protons and neutrons,

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and their constituent quarks
are interacting

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the way they do.

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But with that said, let's
at least think about the

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different types of ways that a
nucleus can essentially decay.

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So let's say I have a
bunch of protons.

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I'll just draw a couple here.

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Some protons there, and I'll
draw some neutrons.

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And I'll draw them in
a neutral-ish color.

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Maybe let me see, like a
grayish would be good.

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So let me just draw some
neutrons here.

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How many protons do I have?

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I have 1, 2, 3, 4, 5, 6, 7, 8.

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I'll do 1, 2, 3, 4, 5,
6, 7, 8, 9 neutrons.

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And so let's say this is the
nucleus of our atom.

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And remember-- and this is, you
know, in the very first

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video I made about the atom--
the nucleus, if you actually

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were to draw an actual atom--
and it's actually very hard to

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drawn an atom because it has
no well-defined boundaries.

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The electron really could be,
you know, at any given moment,

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it could be anywhere.

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But if you were to say, OK,
where is 90% of the time the

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electron is going to be in?

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You'd say, that's the radius,
or that's the

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diameter of our atom.

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We learned in that very first
video that the nucleus is

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almost an infinitesimal portion
of the volume of this

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sphere where the electron
will be 90% of the time.

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And the neat takeaway there
was that, well, most of

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whatever we look at in life
is just open free space.

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All of this is just
open space.

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But I just want to repeat
that because that little

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infinitesimal spot that we
talked about before, where

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even though it's a very small
part of the fraction of the

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volume of an atom-- it's
actually almost all of its

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mass-- that's what I'm zooming
out to this point here.

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These aren't atoms, these
aren't electrons.

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We're zoomed into the nucleus.

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And so it turns out that
sometimes the nucleus is a

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little bit unstable, and it
wants to get to a more stable

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configuration.

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We're not going to go into the
mechanics of exactly what

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defines an unstable nucleus
and all that.

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But in order to get into a
more unstable nucleus,

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sometimes it emits what's called
an alpha particle, or

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this is called alpha decay.

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Alpha decay.

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And it emits an alpha particle,

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which sounds very fancy.

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It's just a collection of
neutrons and protons.

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So an alpha particle is two
neutrons and two protons.

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So maybe these guys, they just
didn't feel like they'd fit in

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just right, so they're a
collection right here.

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And they get emitted.

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They leave the nucleus.

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So let's just think what
happens to an atom when

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something like that happens.

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So let's just say I have some
random element, I'll just call

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it element E.

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Let's say it has p, protons.

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Actually let me do it in the
color of my protons.

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It has p, protons.

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And then it has its atomic mass
number, is the number of

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protons plus the number
of neutrons.

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And do the neutrons
in gray, right?

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So when it experiences
alpha decay, what

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happens to the element?

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Well, its protons are going
to decrease by two.

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So its protons are going
to be p minus 2.

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And then its neutrons are also
going to decrease by two.

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So its mass number's going
to decrease by four.

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So up here you'll have p minus
2, plus our neutrons minus 2,

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so we're going to
have minus 4.

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So your mass is going to
decrease by four, and you're

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actually going to turn
to a new element.

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Remember, your elements
were defined by

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the number of protons.

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So in this alpha decay, when
you're losing two neutrons and

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two protons, but especially the
protons are going to make

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you into a different element.

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So if we call this element 1,
I'm just going to call it,

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we're going to be a different
element now, element 2.

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And if you think about what's
generated, we're emitting

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something that has two protons,

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and it has two neutrons.

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So that its mass is going to be
the mass of the two protons

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and two neutrons.

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So what are we emitting?

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We're emitting something that
has a mass of four.

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So if you look at, what is two
protons and two neutrons?

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I actually don't have the
periodic table on my

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[? head. ?]

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I forgot to cut and paste
it before this video.

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But it doesn't take you long on
the periodic table to find

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an element that has two protons,
and that's helium.

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It actually has an atomic
mass of four.

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So this is actually a helium
nucleus that gets emitted with

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alpha decay.

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This is actually a
helium nucleus.

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And because it's a helium
nucleus and it has no

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electrons to bounce off its two
protons, this would be a

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helium ion.

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So essentially it has
no electrons.

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It has two protons so it
has a plus 2 charge.

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So an alpha particle is really
just a helium ion, a plus 2

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charged helium ion that is
spontaneously emitted by a

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nucleus just to get to
a more stable state.

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Now that's one type of decay.

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Let's explore the other ones.

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So let me draw another
nucleus here.

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I'll draw some neutrons.

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I'll just draw some protons.

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So it turns out sometimes that
a neutron doesn't feel

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comfortable with itself.

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It looks at what the protons do
on a daily basis and says,

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you know what?

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For some reason when I look into
my heart, I feel like I

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really should be a proton.

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If I were a proton, the entire
nucleus would be a little bit

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more stable.

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And so what it does is, to
become a proton-- remember, a

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neutron has neutral charge.

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So what it does is, it
emits an electron.

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And I know you're saying, Sal,
you know, that's crazy, I

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didn't even know neutrons
had electrons in

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them, and all of that.

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And I agree with you.

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It is crazy.

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And one day we'll study
all of what exists

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inside of the nucleus.

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But let's just say that it
can emit an electron.

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So this emits an electron.

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And we signify that with its--
roughly its mass is zero.

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We know an electron really
doesn't have a zero mass, but

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we're talking about
atomic mass units.

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If the proton is one, an
electron is 1/1,836 of that.

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So we just round it.

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We say it has a mass of zero.

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Its mass really isn't zero.

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And its charge is minus 1.

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It's atomic, you can kind
of say its atomic

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number's minus 1.

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So it emits an electron.

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And by emitting an electron,
instead of being neutral, now

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it turns into a proton.

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And so this is called
beta decay.

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And a beta particle is really
just that emitted electron.

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So let's go back to our little
case of an element.

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It has some number of protons,
and then it has

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some number of neutrons.

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So you have the protons and
the neutrons, then you get

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your mass number.

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When it experiences beta
decay, what happens?

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Well, are the protons changed?

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Sure, we have one more proton
than we had before.

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Because our neutron
changed into one.

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So now our protons are plus 1.

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Has our mass number changed?

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Well let's see.

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The neutrons goes down
by one but your

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protons go up to by one.

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So your mass number
will not change.

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So it's still going
to be p plus N.

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so your mass stays the same,
unlike the situation with

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alpha decay, but your
element changes.

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Your number of protons
changes.

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So now, once again, you're
dealing with a new element in

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beta decay.

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Now, let's say we have
the other situation.

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Let's say we have a situation
where one of these protons

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looks at the neutrons and
says, you know what?

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I see how they live.

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It's very appealing to me.

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I think I would fit in better,
and our community of particles

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within the nucleus would
be happier if

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I too were a neutron.

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We'd all be in a more
stable condition.

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So what they do is, that little
uncomfortable proton

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has some probability of
emitting-- and now this is a

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new idea to you-- a positron,
not a proton.

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It emits a positron.

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And what's a positron?

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It's something that
has the exact

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same mass as an electron.

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So it's 1/1836 of the
mass of a proton.

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But we just write a zero there
because in atomic mass units

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it's pretty close to zero.

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But it has a positive charge.

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And it's a little confusing,
because they'll

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still write e there.

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Whenever I see an e, I
think an electron.

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But no, they say e because it's
kind of like the same

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type of particle, but instead
of having a negative charge,

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it has a positive charge.

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This is a positron.

00:10:02.080 --> 00:10:04.980


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And now we're starting to get
kind of exotic with the types

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of particles and stuff
we're dealing with.

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But this does happen.

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And if you have a proton that
emits this particle, that

00:10:15.920 --> 00:10:19.370
pretty much had all of its
positive charge going with it,

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this proton turns
into a neutron.

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And that is called positron
emission.

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Positron emission is usually
pretty easy to figure out what

00:10:31.350 --> 00:10:33.510
it is, because they call
it positron emission.

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So if we start with the same E,
it has a certain number of

00:10:37.880 --> 00:10:41.500
protons, and a certain
number of neutrons.

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What's the new element
going to be?

00:10:43.190 --> 00:10:46.060
Well it's going to lose
a proton. p minus 1.

00:10:46.060 --> 00:10:47.770
And that's going to be turned
into a neutron.

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So p is going to
go down by one.

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N is going to go up by one.

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So that the mass of the whole
atom isn't going to change.

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So it's going to be p plus N.

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But we're still going to have
a different element, right?

00:11:00.500 --> 00:11:03.230
When we had beta decay,
we increased

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the number of protons.

00:11:04.150 --> 00:11:06.700
So we went, kind of, to the
right in the periodic table or

00:11:06.700 --> 00:11:09.070
we increased our, well,
you get the idea.

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When we do positron emission,
we decreased

00:11:12.440 --> 00:11:14.700
our number of protons.

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And actually I should
write that here in

00:11:16.300 --> 00:11:17.510
both of these reactions.

00:11:17.510 --> 00:11:20.460
So this is the positron
emission, and I'm left over

00:11:20.460 --> 00:11:22.060
with one positron.

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And in our beta decay, I'm left
over with one electron.

00:11:29.430 --> 00:11:30.670
They're written the
exact same way.

00:11:30.670 --> 00:11:32.660
You know this is an electron
because it's a minus 1 charge.

00:11:32.660 --> 00:11:33.890
You know this is a positron
because it

00:11:33.890 --> 00:11:35.810
has a plus 1 charge.

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Now there's one last
type of decay that

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you should know about.

00:11:39.140 --> 00:11:42.810
But it doesn't change the number
of protons or neutrons

00:11:42.810 --> 00:11:43.970
in a nucleus.

00:11:43.970 --> 00:11:46.940
But it just releases a ton of
energy, or sometimes, you

00:11:46.940 --> 00:11:48.350
know, a high-energy proton.

00:11:48.350 --> 00:11:50.160
And that's called gamma decay.

00:11:50.160 --> 00:11:52.510
And gamma decay means that these
guys just reconfigure

00:11:52.510 --> 00:11:52.795
themselves.

00:11:52.795 --> 00:11:54.460
Maybe they get a little
bit closer.

00:11:54.460 --> 00:11:57.990
And by doing that they release
energy in the form of a very

00:11:57.990 --> 00:12:03.180
high wavelength electromagnetic
wave. Which is

00:12:03.180 --> 00:12:05.820
essentially a gamma, you could
either call it a gamma

00:12:05.820 --> 00:12:08.230
particle or gamma ray.

00:12:08.230 --> 00:12:09.450
And it's very high energy.

00:12:09.450 --> 00:12:11.720
Gamma rays are something you
don't want to be around.

00:12:11.720 --> 00:12:15.460
They're very likely
to maybe kill you.

00:12:15.460 --> 00:12:17.130
Everything we did, I've said
is a little theoretical.

00:12:17.130 --> 00:12:20.000
Let's do some actual problems,
and figure out what type of

00:12:20.000 --> 00:12:21.750
decay we're dealing with.

00:12:21.750 --> 00:12:24.400
So here I have 7-beryllium
where seven

00:12:24.400 --> 00:12:26.900
is its atomic mass.

00:12:26.900 --> 00:12:30.520
And I have it being converted
to 7-lithium So

00:12:30.520 --> 00:12:31.440
what's going on here?

00:12:31.440 --> 00:12:36.000
My beryllium, my nuclear mass
is staying the same, but I'm

00:12:36.000 --> 00:12:42.240
going from four protons
to three protons.

00:12:42.240 --> 00:12:45.130
So I'm reducing my number
of protons.

00:12:45.130 --> 00:12:46.840
My overall mass hasn't
changed.

00:12:46.840 --> 00:12:49.100
So it's definitely
not alpha decay.

00:12:49.100 --> 00:12:50.960
Alpha decay was, you know,
you're releasing a whole

00:12:50.960 --> 00:12:52.770
helium from the nucleus.

00:12:52.770 --> 00:12:54.960
So what am I releasing?

00:12:54.960 --> 00:12:57.410
I'm kind of releasing one
positive charge, or I'm

00:12:57.410 --> 00:12:58.560
releasing a positron.

00:12:58.560 --> 00:13:00.940
And actually I have this
here in this equation.

00:13:00.940 --> 00:13:04.040
This is a positron.

00:13:04.040 --> 00:13:07.140
So this type of decay of
7-beryllium to 7-lithium is

00:13:07.140 --> 00:13:09.760
positron emission.

00:13:09.760 --> 00:13:10.830
Fair enough.

00:13:10.830 --> 00:13:12.400
Now let's look at
the next one.

00:13:12.400 --> 00:13:19.870
We have uranium-238 decaying
to thorium-234.

00:13:19.870 --> 00:13:25.140
And we see that the atomic mass
is decreasing by 4, minus

00:13:25.140 --> 00:13:28.910
4, and you see that your atomic
numbers decrease, or

00:13:28.910 --> 00:13:31.270
your protons are decreasing,
by 2.

00:13:31.270 --> 00:13:33.810
So you must be releasing,
essentially, something that

00:13:33.810 --> 00:13:37.390
has an atomic mass of four,
and a atomic number

00:13:37.390 --> 00:13:39.680
of two, or a helium.

00:13:39.680 --> 00:13:42.210
So this is alpha decay.

00:13:42.210 --> 00:13:46.100
So this right here is
an alpha particle.

00:13:46.100 --> 00:13:48.400
And this is an example
of alpha decay.

00:13:48.400 --> 00:13:51.110
Now you're probably saying, hey
Sal, wait, something weird

00:13:51.110 --> 00:13:51.850
is happening here.

00:13:51.850 --> 00:13:56.630
Because if I just go from 92
protons to 90 protons, I still

00:13:56.630 --> 00:13:59.430
have my 92 electrons out here.

00:13:59.430 --> 00:14:02.750
So wouldn't I now have
a minus 2 charge?

00:14:02.750 --> 00:14:08.270
And even better, this helium I'm
releasing, it doesn't have

00:14:08.270 --> 00:14:09.090
any electrons with it.

00:14:09.090 --> 00:14:10.390
It's just a helium nucleus.

00:14:10.390 --> 00:14:12.700
So doesn't that have
a plus 2 charge?

00:14:12.700 --> 00:14:15.180
And if you said that, you would
be absolutely correct.

00:14:15.180 --> 00:14:19.510
But the reality is that right
when this decay happens, this

00:14:19.510 --> 00:14:22.290
thorium, it has no reason
to hold on to those two

00:14:22.290 --> 00:14:25.050
electrons, so those two
electrons disappear and

00:14:25.050 --> 00:14:26.840
thorium becomes neutral again.

00:14:26.840 --> 00:14:30.480
And this helium, likewise,
it is very quick.

00:14:30.480 --> 00:14:33.040
It really wants two electrons
to get stable, so it's very

00:14:33.040 --> 00:14:36.880
quick to grab two electrons out
of wherever it's bumping

00:14:36.880 --> 00:14:38.460
into, and so that
becomes stable.

00:14:38.460 --> 00:14:40.305
So you could write
it either way.

00:14:40.305 --> 00:14:42.250
Now let's do another one.

00:14:42.250 --> 00:14:43.500
So here I have iodine.

00:14:43.500 --> 00:14:45.820


00:14:45.820 --> 00:14:46.670
Let's see what's happening.

00:14:46.670 --> 00:14:51.020
My mass is not changing.

00:14:51.020 --> 00:14:53.790
So I must just have protons
turning into neutrons or

00:14:53.790 --> 00:14:55.560
neutrons turning into protons.

00:14:55.560 --> 00:14:58.810
And I see here that I
have 53 protons, and

00:14:58.810 --> 00:15:00.800
now I have 54 protons.

00:15:00.800 --> 00:15:04.060
So a neutron must have
turned into a proton.

00:15:04.060 --> 00:15:06.830
A neutron must have
gone to a proton.

00:15:06.830 --> 00:15:09.160
And the way that a neutron
goes to a proton is by

00:15:09.160 --> 00:15:11.620
releasing an electron.

00:15:11.620 --> 00:15:13.360
And we see that in this
reaction right here.

00:15:13.360 --> 00:15:16.880
An electron has been released.

00:15:16.880 --> 00:15:19.130
And so this is beta decay.

00:15:19.130 --> 00:15:20.380
This is a beta particle.

00:15:20.380 --> 00:15:25.580


00:15:25.580 --> 00:15:26.750
And that same logic holds.

00:15:26.750 --> 00:15:32.780
You're like, hey wait, I just
went from 53 to 54 protons.

00:15:32.780 --> 00:15:34.440
Now that I have this extra
proton, won't I have a

00:15:34.440 --> 00:15:35.750
positive charge here?

00:15:35.750 --> 00:15:36.480
Well you would.

00:15:36.480 --> 00:15:40.810
But very quickly this might--
probably won't get these exact

00:15:40.810 --> 00:15:42.740
electrons, there's so many
electrons running around-- but

00:15:42.740 --> 00:15:45.950
it'll grab some electrons from
some place to get stable, and

00:15:45.950 --> 00:15:47.080
then it'll be stable again.

00:15:47.080 --> 00:15:48.890
But you're completely right in
thinking, hey, wouldn't it be

00:15:48.890 --> 00:15:51.690
an ion for some small
amount of time?

00:15:51.690 --> 00:15:52.900
Now let's do one more.

00:15:52.900 --> 00:15:57.210
So we have to 222-radon-- it
has atomic number of 86--

00:15:57.210 --> 00:16:01.720
going to 218-polonium, with
atomic number of 84.

00:16:01.720 --> 00:16:03.540
And this actually is an
interesting aside.

00:16:03.540 --> 00:16:08.380
Polonium is named after Poland,
because Marie Curie,

00:16:08.380 --> 00:16:11.220
she-- At the time Poland, this
was at the turn of the last

00:16:11.220 --> 00:16:15.120
century, around the end of the
1800's, Poland didn't exist as

00:16:15.120 --> 00:16:15.910
a separate country.

00:16:15.910 --> 00:16:19.540
It was split between Prussia,
Russia, and Austria.

00:16:19.540 --> 00:16:21.590
And they really wanted let
people know that, hey, you

00:16:21.590 --> 00:16:24.000
know, we think we're
one people.

00:16:24.000 --> 00:16:27.170
So they discovered that when,
you know, radon decayed it

00:16:27.170 --> 00:16:27.730
formed this element.

00:16:27.730 --> 00:16:31.430
And they named it after their
motherland, after Poland.

00:16:31.430 --> 00:16:33.880
It's the privileges of
discovering new elements.

00:16:33.880 --> 00:16:35.090
But anyway, back
to the problem.

00:16:35.090 --> 00:16:35.930
So what happened?

00:16:35.930 --> 00:16:39.210
Our atomic mass went
down by four.

00:16:39.210 --> 00:16:41.430
Our atomic number went
down by two.

00:16:41.430 --> 00:16:44.580
Once again, we must have
released a helium particle.

00:16:44.580 --> 00:16:47.070
A helium nucleus, something
that has an atomic mass of

00:16:47.070 --> 00:16:51.160
four, and an atomic
number of two.

00:16:51.160 --> 00:16:52.100
And so there we are.

00:16:52.100 --> 00:16:55.950
So this is alpha decay.

00:16:55.950 --> 00:16:57.810
We could write this as
a helium nucleus.

00:16:57.810 --> 00:16:59.145
So it has no electrons.

00:16:59.145 --> 00:17:00.820
We could even say immediately
that this would have a

00:17:00.820 --> 00:17:02.990
negative charge, but
then it loses