This lecture is about the Geiger-Marsden experiment,
which was an experiment carried out in 1911 to determine the structure of atoms.
The experiment disproved the plum pudding model of an atom, which we'll talk about in a moment,
and showed that atoms have a very small dense positively charged nucleus and are otherwise
mostly empty space with some electrons floating around. The scientists performing the experiment
shot alpha particles at a piece of gold foil and observed the directions that the particles
scattered. This is also sometimes called the Rutherford-Geiger-Marsden experiment
since Rutherford directed the experiment while Geiger and Marsden carried it out. Before the
Geiger-Marsden experiment, scientists believed in something called the plum pudding model of
an atom. They knew that there were positive and negative charges in an atom, and in non-ionized
atoms the charges added together to make 0, but the plum pudding model of an atom said
that an atom was small electrons embedded inside of a uniform positively charged mass
(something like this picture right here where you have electrons stuck in a spherical
low-density positively charged mass). The reason it's called the plum pudding model is that this
physically looks like a plum pudding with little pieces of fruit embedded in the cake, and the
Geiger-Marsden experiment allowed us to move from this model to a more correct model of an atom
called the nuclear model where the positive charge of an atom exists in a very small dense nucleus
surrounded mostly by empty space and electrons. So the experiment showed that the plum pudding
model was probably not correct and hinted at the nuclear model instead. In the experiment
itself scientists took a very thin film of gold foil and placed it inside of a circular screen.
This is an overhead view of the experiment. I want you to imagine that when you're looking at this
you're looking physically down at a table that you're standing over, and the circular gray screen
is resting on the table horizontally, and the gold foil is placed in the middle. The reason why the
scientists used gold specifically is that gold can hold its structure while still being incredibly
thin. The scientists doing the experiment were pretty confident that the gold foil itself was
actually probably only 2 or 3 lines of atoms thick, so any interference with these atoms would
be the result of interference with just a single atom, or maybe 2 atoms at the most, so they knew
that whatever happened in the experiment would be the result of interference with just a single atom
rather than the result of the interference of very large amounts of atoms, because there
could be other factors that were affecting the experiment that we didn't know about,
but if they only dealt with individual atoms they could get a pretty clear idea of the
shape of the atom. Scientists set up a source of alpha radiation: just a radioactive substance
inside of a lead box, and they created a very small opening in the box, and the substance would
emit alpha particles in every direction but the only alpha particles that could actually escape
the box would have to be going straight through that hole so they would continue along their
same path and eventually go straight at the gold foil. Without the gold foil there, they knew that
what would happen is an alpha particle would be emitted, hit the screen, and they would be able to
see on the screen exactly where the alpha particle impacted the screen. There would be a little mark
on the screen that appeared. So I'll use a green dot to represent what those marks looked like.
We now know that alpha particles are 2 protons and 2 neutrons, so they're positively charged.
At the time of the experiment it was known that the alpha particle is small dense and positively
charged, but scientists did not yet know that it's a proton and a neutron because they didn't even
know that protons and neutrons existed at the time. So they were trying to confirm the plum
pudding model of the atom, and if the plum pudding model were true, one of two things would happen
when you sent a lot of alpha particles through an atom. One possibility is that all the alpha
particles would move through the atom. Because scientists assumed that the atom was very low
density, the alpha particles could probably just effortlessly move through to the other side
because they were much smaller and denser. The important thing here though is that the
plum pudding model predicted that the atom was uniform. There's no small dense nucleus in the
center surrounded by empty space. Instead, the entire atom is just the same low density object,
and so no matter what part of the atom a particle moves through, the top middle and bottom paths
that the alpha particles took should all be the same going through the atom regardless of which
specific part of the atom they traveled through. The other possibility was that maybe the atom
was actually denser than alpha particles, or dense enough for them to be repelled,
so maybe all the alpha particles would be repelled back this way. This wasn't a serious assumption,
but it was obviously a possibility in the plum pudding model as well, because if the model is
true there's a uniform density, so wherever the alpha particle impacted the atom shouldn't matter.
Same density everywhere. So it should always bounce off or it should always go through.
So if the plum pudding model were true we would expect one of these behaviors or the other,
and it was known that the alpha particles were small and dense and that the atoms were much
larger and less dense, so it was assumed that the top example would happen. So based on those
assumptions this is what we would expect to see if the plum pudding model were true:
if we shot a lot of alpha particles at the gold foil we would expect their imprints
on the screen to appear here. The reason why we would expect them to see them here
is that if all the alpha particles were able to effortlessly move through the atom,
we would expect that all the alpha particles would follow this path through the gold foil,
because the gold foil is just made of individual atoms, so we would expect them to appear there on
the other side of the foil. The other possibility if the atom were more dense is that this pattern
would appear, because if the atom were more dense all of the alpha particles would be rebounded off
of the atoms in random directions and so the alpha particles set here would follow this pattern here
and impact the other side of the screen in random spots. So we would expect one or the other pattern
if the plum pudding model were true, but that's not what actually happened. Instead we got this
pattern here. You can see that most of the alpha particles did hit the other side, but there are
a few that are off in strange directions, and a few that actually bounced off the gold foil
and exist on the left side of the gold foil. This hints at a pattern of the alpha particles
moving through the atom like this, because when they came out of the source of the radiation
most of them moved straight through the gold foil, but a few bounced off in random directions,
and a few were even bounced backwards in the opposite direction like that.
So a majority moved through, but you had a few that bounced off in other directions,
so if you look up at my diagram of the atom, what this implies is that most of the atom is
very easy for particles to move through, but there must be some very small very dense spot
where alpha particles are bounced off that deflects alpha particles and pushes them away,
and it's dense enough for alpha particles not to just effortlessly move through it. It was known
that alpha particles were positively charged, so it was assumed that the small dense nucleus
repelling them must also be positively charged, so this pattern hinted that the plum pudding model
was wrong, and instead a nuclear model of the atom was correct where most of the atom was empty
space that was very easy for alpha particles to move through, but a very small portion of
the atom must be a small dense positively charged region that repels alpha particles.
So this is how we got from the plum pudding model of the atom to the nuclear model of an atom,
and that's how the Geiger-Marsden experiment helped us progress from one to the other.