馃攳
Writing nuclear equations for alpha, beta, and gamma decay | Chemistry | Khan Academy - YouTube
Channel: unknown
[1]
- [Voiceover] Let's look at three types
[2]
of radioactive decay,
[3]
and we'll start with alpha decay.
[5]
In alpha decay, an alpha particle
[7]
is ejected from an unstable nucleus,
[10]
so here's our unstable
nucleus, uranium-238.
[15]
An alpha particle has the same
[16]
composition as a helium nucleus.
[18]
We saw the helium nucleus
in the previous video.
[22]
There are two protons
[23]
in the helium nucleus and two neutrons.
[26]
So I go ahead and draw
in my two neutrons here.
[29]
Since there are two protons,
[31]
the charge of an alpha
particle is two plus.
[34]
So for representing an
alpha particle in our
[37]
nuclear equation, since an alpha particle
[40]
has the same composition
as a helium nucleus,
[42]
we put an He in here,
[44]
and it has two positive charges,
[47]
so we put a two down here,
[49]
and then a total of four nucleons,
[51]
so we put a four here.
[53]
Trying to figure out the other product
[55]
from our nuclear equation,
[56]
I know nucleons are conserved,
[58]
so if I have 238 nucleons on the left,
[61]
I need 238 nucleons on the right.
[64]
Well, I have four from my alpha particle,
[66]
so I need 234 more.
[69]
So 234 plus four gives me a
total of 238 on the right,
[73]
and so therefore nucleons
are conserved here.
[76]
In terms of charge, I know
charge is also conserved.
[79]
On the left, I know I have 92 protons,
[82]
so 92 positive charges on the left.
[85]
I need 92 positive charges on the right.
[88]
We already have two positive charges
[89]
from our alpha particle,
[91]
and so we need 90 more.
[93]
So we need 90 positive charges.
[95]
We need an atomic number here of 90.
[98]
The identity of the other product,
[101]
just look it up here at our table,
[103]
find atomic number of 90,
[105]
and you'll see that's thorium here.
[107]
So thorium-234 is our other product.
[111]
So we think about what's
happening visually,
[114]
we're starting off with a uranium nucleus
[118]
which is unstable, it's going
to eject an alpha particle,
[121]
so an alpha particle is
ejected from this nucleus,
[125]
so we're losing this alpha particle,
[127]
and what's left behind
is this thorium nucleus.
[132]
So this is just a visual representation
[134]
of what's going on here,
in our nuclear equation.
[140]
Let's do beta decay.
[142]
So in beta decay, an electron
is ejected from the nucleus.
[147]
We saw in the previous video that
[149]
you represent an electron,
[150]
since it has a negative one charge,
[152]
you put a negative one down here,
[154]
it's not a proton, nor is it a neutron,
[156]
so we put a zero here.
[157]
So here's our electron and an electron
[160]
ejected from the nucleus
is called a beta particle.
[162]
We could put a beta here,
and it's an electron,
[166]
so a negative one charge,
and then a zero here.
[170]
If a beta particle is
ejected from the nucleus
[172]
of a thorium-234, so we're
starting with thorium-234,
[177]
this nucleus ejects a beta particle,
[180]
so we go ahead and put
a beta particle in here,
[182]
so zero and negative one,
what else is produced here?
[186]
What else do we make?
[187]
Well, once again, the number of nucleons
[190]
is conserved, so I have
234 nucleons on the left,
[193]
I need 234 on the right.
[195]
I have a zero here, so
I need 234 nucleons.
[199]
Charge is also conserved,
so I have 90 positive
[203]
charges on the left, I have 90 protons.
[206]
On the right, I have a
negative charge here,
[209]
so I have a negative one charge,
[210]
and so I must need 91 positive charges,
[213]
because 91 positive charges
[215]
and one negative charge gives me
[217]
90 positive charges on the right.
[220]
So I need an atomic number of 91.
[225]
If you look at the periodic table,
[227]
and you find the atomic number of 91,
[230]
you'll see that this is protactinium.
[233]
So we're going to make
protactinium here, so Pa.
[236]
What is happening in beta decay?
[239]
Let's look at it in a
little bit more detail.
[242]
We already talked about
the number of protons,
[244]
so we have 90 protons on the left,
[247]
how many neutrons do we have?
[249]
Well, 234 minus 90, 234 minus 90
[253]
gives us the number of neutrons.
[254]
That's 144 neutrons.
[258]
On the right, we have 91 protons,
[262]
how many neutrons do we have?
[264]
Well, that'd be 234 minus 91.
[266]
So 234 minus 91 gives us 143 neutrons.
[272]
So we went from 144 neutrons on the left
[275]
to 143 neutrons on the right,
[278]
and we went from 90 protons on the left,
[282]
to 91 protons on the right.
[284]
So we lost a neutron,
and we gained a proton.
[290]
You could think about the
neutron turning into a proton,
[294]
and this is an oversimplified
way of thinking about it.
[297]
Let's go ahead and write that down here.
[299]
So a neutron turning into a proton.
[301]
So a neutron has no charge,
so we put a zero here.
[306]
And a neutron is a nucleon,
so we put a one right here.
[310]
So a neutron is turning into a proton,
[313]
so let's go ahead and
write our proton here.
[314]
A proton has a plus one charge,
[316]
and it's a nucleon so we put a one here.
[320]
When we think about what else is made,
[322]
we know that nucleons are conserved,
[325]
so we have one nucleon on the left,
[326]
one nucleon on the right.
[329]
Therefore, we would have a zero here.
[332]
In terms of charge, if we
have zero charge on the left,
[336]
plus one on the right, we
need negative one right here.
[340]
This of course represents the electron,
[342]
so this is the electron that's
ejected from the nucleus.
[345]
This is our beta particle.
[349]
And also actually,
something else is produced.
[351]
You're also going to
make an anti-neutrino,
[355]
and that's just really
not part of this video,
[357]
so we'll just ignore it for now.
[359]
So a neutron has turned into a proton,
[363]
and we're also getting a beta particle
[366]
ejected from the nucleus.
[367]
When this conversion, this process
[369]
is actually governed by the weak force,
[371]
the weak interaction, so there's a lot
[373]
of stuff going on in the nucleus
[375]
which we just won't
get into in this video.
[378]
The important thing is to be able to look
[381]
at a nuclear equation, recognize it
[384]
as beta decay, and be able to write
[387]
everything in your nuclear equation.
[390]
Let's do one more type of decay.
[392]
This is gamma decay.
[395]
Gamma rays are given off,
[397]
and a gamma ray has no charge and no mass;
[401]
it's pretty much just energy,
if you think about it.
[404]
These are pretty easy decay problems.
[407]
Let's start with technetium-99m,
[411]
and the m right here
stands for metastable,
[414]
which means a nucleus
in its excited state,
[417]
so a nucleus in its excited state,
[420]
so it has more energy.
[423]
It's going to give off a gamma ray,
[426]
so let's go ahead and draw in our
[427]
gamma ray here, so zero and zero.
[431]
Since we're dealing with zeroes,
[433]
so these zeroes aren't
going to affect our numbers,
[436]
so if we start with nucleons,
[438]
we have 99 nucleons on the left,
[440]
we're going to have 99
nucleons on the right.
[443]
And in terms of charges,
[445]
we have 43 positive charges on the left,
[448]
we need 43 positive charges on the right.
[450]
And since the atomic
number isn't changing,
[453]
it's 43 on the left, it's 43 on the right,
[456]
we're dealing with technetium here.
[458]
It's still technetium; it's
just in the ground stage.
[460]
It's no longer in the excited state.
[463]
It's in the ground state.
[465]
It's given off energy in the form
[467]
of gamma rays in this example here.
[470]
So technetium-99m is actually used
[475]
in several medical imaging
and diagnostic procedures,
[478]
because we have ways of
measuring the gamma radiation,
[482]
and so this is very useful in medicine.
Most Recent Videos:
You can go back to the homepage right here: Homepage