- If you search online for how the nucleus was discovered,
you'll probably bump into Rutherford's gold foil experiment,
where a beam of alpha particles hit a piece of gold
and then the position of the alpha particle
is found on a screen that is wrapped around the gold.
Now, there was experiment like this was done in 1913
but it wasn't done by Rutherford
nor did it inspire Rutherford to discover the nucleus.
In fact, it was done to verify Rutherford's theory
about the nucleus
that he had published two years earlier.
The real influential experiment was done in 1909
and involved eight different metals,
not just gold, but was far simpler.
So simple, in fact,
that Rutherford gave it to an undergrad as busywork.
Ready for the real story?
Let's go.
♪ Electricity, electricity ♪
It all started in 1899.
That was when Rutherford,
who was a graduate student at Cambridge,
noticed that the radiation from uranium was complex
in that some was easy to block and some wasn't.
He labeled the easiest blocked radiation as alpha radiation
and the other radiation as beta.
Rutherford then got a job as a professor
at McGill University,
where he had, as he put it, "a swell lab"
and proceeded to do a lot of amazing work
that I talked about in my last video.
You should check it out.
In 1904, an Australian named William Bragg
published his theory
that alpha particles travel through thin solids
without any deviation.
In general, Rutherford thought very highly of Bragg's work,
but in this one point, he differed.
See, Rutherford thought everything was full of electrons,
teeny tiny little electrified particles
that were so small that you could get really close to them
and therefore feel a pretty strong electrical force.
J.J. Thomson, Rutherford's advisor at Cambridge,
had discovered electrons,
which he called corpuscles, back in 1897.
And by 1904, Thomson concluded that,
"atoms consist of a number
of negatively electrified corpuscles
enclosed in a sphere of uniform positive electrification."
This was colloquially called
the plum pudding model of atoms,
where the electrons were the raisins,
or if you are a Brit, the plums,
in a positive pudding.
At around this time, Rutherford had became convinced
that alpha particles were positive,
had twice the charge of an electron,
and around 7,000 times the mass.
Despite this, Rutherford thought
that the tiny light electrons
could push heavy alpha particles electrically
because they could get so close to each other,
just not by much.
In 1906, he published an experimental proof of his theory.
Rutherford put a small piece of radioactive wire in a groove
and then had the beam of alpha particles from the wire
go through a small slit
and illuminate a photographic plate.
He then covered half of the slit with a mica screen.
The beam that went through the mica
was a little bit more spread out
than the beam that didn't by about 2%.
Rutherford concluded that,
"Such a result brings out clearly the fact
that the atoms of matter
must be the seat of very intense electrical forces,
a deduction in harmony
with the electronic theory of matter."
In 1907, Rutherford,
who felt left out in the "colonies" of Canada,
got a job at the University of Manchester
where he met a young German scientist named Hans Geiger.
Geiger and Rutherford then determined
that a screen made out of phosphorescent zinc sulfide
would glow when hit with an alpha particle,
and only alpha particles,
in a process called scintillation.
Rutherford hated scintillation
and left it to the younger and more patient Geiger,
writing a friend that,
"Geiger is a demon at the work of counting scintillations
and could count at intervals for a whole night
without destroying his equanimity.
I damned vigorously and retired after two minutes."
There was another problem with scintillation,
"the doubt" as Rutherford put it,
"that once arises
whether every alpha particle produces a scintillation."
Therefore, Rutherford turned to "an electric method".
Now an alpha particle have a charge of 2e
or twice the charge on an electron,
which is very, very small,
way smaller than can be measured by an electrometer.
Luckily, Rutherford's friend from his time with JJ Thomson,
a man named John Townsend,
had found that if he put a gas at low pressure
under really high voltage until it almost sparked,
just a little charge would create an avalanche of charges
and so a little charge could be magnified.
In 1908, Rutherford wrote,
"Under such conditions,
the ionization produced in the gas by the alpha particle
is magnified 2,000 times by collision.
The effect of each alpha particle
is marked enough to show an audience."
By July of 1908,
Rutherford and Geiger published that,
"The number of scintillations
is within the limit of experimental error,
equal to the number of alpha particles falling upon it,
as counted by the electric method."
Around the same time as the electric experiment,
Geiger was also studying the number of alpha particles
that were emitted by a substance by scintillations
when he noticed that the alpha particles
were a little bit scattered by air.
Geiger remarked upon it to Rutherford
and Rutherford remembered his experiment with mica screen.
He then told Geiger
to redo the experiment with the scintillation method
so they could get
an exact distribution of the alpha particles
as they went through a thin material.
Geiger thus put radium in a small lead evacuated cone
where one end was covered in thin mica
that let the alpha particles escape.
He then had the beam of particles go through a small slit
that could be covered with foil
and then examined the alpha particles
that hit a phosphorescent screen with a microscope
that could be moved up or down by about 10 millimeters.
In this way, Geiger produced smooth curves
of the position of alpha particle
diverted by thin metals.
The next year,
a 20 year old undergraduate named Ernest Marsden
joined the research group.
What happened next is best described by Rutherford himself.
"One day Geiger came to me and said,
'Don't you think that young Marsden
ought to begin a small research?'
Now I had thought that too, so I said,
'Why not let him see if any alpha particles
can be scattered through a large angle?'
I may tell you in confidence
that I did not believe that they would be,
since we knew that the alpha particle
was a very fast massive particle,
with a great deal of energy.
Then I remember two or three days later
Geiger coming to me in great excitement and saying,
'We have been able to get
some of the alpha particles coming backwards.'"
So what did Marsden do?
Well, he used the same cone
of radioactive material as before
but had it hit a foil of metal at an angle
and then have the reflected alpha particles hit a screen
that was examined by a microscope.
He also placed a lead barrier
between the screen and the radioactive source
so that everything he saw had to reflect off the foil.
Geiger and Marsden then experimented
with eight different metals
and found that a few alpha particles
would bounce off thin metals,
where the heavier the metal,
the more alpha particles would reflect.
This is not to imply
that a lot of alpha particles were reflected.
In fact, in the same experiment,
they found that only about one in 8,000 alpha particles
would reflect off a piece of platinum.
Still, why would any heavy alpha particles
reflect off a thin metal?
It didn't make sense.
Rutherford recalled, "It was quite the most incredible event
that has ever happened to me in my life.
It was almost as incredible as if you fired a 15-inch shell
at a piece of tissue paper
and it came back and hit you!"
Rutherford thought about it for well over a year,
and by December of 1910 wrote a friend,
"I think I can devise an atom much superior to JJ's.
It will account for the reflected alpha particles
observed by Geiger,
and generally I think will make a fine working hypothesis."
Rutherford's logic according to his own words was
"I realized that this scattering backwards
must be the result of a single collision,
and when I made calculations,
I saw that it was impossible
to get anything of that order of magnitude
unless you took a system
in which the greater part of the mass of the atom
was concentrated in a minute nucleus."
On March 7, 1911,
Rutherford published an article
where the atom contained of
"a positive charge Ne at its center,
surrounded by a distribution of negative electricity Ne
uniformly distributed within a sphere of radius R."
Rutherford then concluded
from the experimental results of Geiger and Marsden
that the platinum nucleus had a charge of about 100e
or 100 times the charge of an electron,
and the nucleus was about 10,000 times smaller
than the atom itself.
He was startlingly close.
We now think that platinum has a charge 78e
and the nucleus is around 100,000 times smaller
than the atom itself.
That is ridiculously small.
If an atom were expanded to the size of a cathedral,
the nucleus would be only about the size of a fly,
but a fly that holds 99.9997% of the weight.
Or to put it the other way,
atoms, and therefore all matter,
contains 99.999999999999999% material
that's only filled with only 0.0003% of the mass.
So basically, almost everything is filled
with almost nothing.
So for all almost nothing,
why don't I just fall through the floor?
It's because of the teeny tiny electrons in everything.
See, the electrons in the atoms in your shoes
push against the electrons in the atoms in the floor
when they get close enough to each other.
You are not really touching the floor, or anything really.
You are actually hovering above it just a tiny bit,
due to electrical forces.
Crazy, huh?
Back in 1913,
Geiger and Marsden did conduct a gold foil experiment
although it wasn't done with a wrap around screen
but instead a microscope coated with a screen
that could be moved in a full circle.
That gold foil experiment validated Rutherford's conclusions
and proved that the nucleus was positive,
something that Rutherford was pretty convinced about,
but it was nice to have validation.
Rutherford's model created a lot of interesting questions.
One of the biggest had to do
with the positive nucleus and the negative electrons.
Opposite charges attract,
what keeps the negative electrons
from being sucked in the positive nucleus?
Even if they're are spinning
around the nucleus like planets,
they are accelerating charges,
spinning is a form of acceleration,
and according to Maxwell's laws,
they should be creating electromagnetic waves,
which would cause the electrons to lose energy
and then spiral into the nucleus.
In other words, according to the laws of classical physics,
Rutherford's atom should just implode.
But we exist.
So either Rutherford was wrong
or the classical physics doesn't work on the atomic scale.
This is why in July of 1913,
a young Danish man named Neil Bohr
who was working for Earnest Rutherford
created the first quantum mechanical view of the atom,
the Bohr model.
In this model, Bohr used the results from a 1900 paper
about blackbody radiation written by a man named Max Planck.
But to tell that story,
and the story of quantum mechanics in general,
I first want to take a step back
and talk about the man
who came up with the idea of blackbody radiation
in the first place.
This is a fabulous story about a tiny man
and his large spacey friend
who transformed our world with a prism
and literally got gold from the sun, literally.
How Kirchhoff and Bunsen discovered spectroscopy,
the systematic study of the color from elements,
and that led to discover blackbody radiation
is next time on the lightning tamers.
Thanks for watching my video.
If you haven't watched my last video about Rutherford,
you should definitely check it out.
Also, my next video about Bunsen and Kirchhoff
is my favorite story of all of science.
The science is great, it's simple, it's weird, it's cool,
they're both such interesting characters,
it's just fabulous.
You probably should check it out.
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Bye now.
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