The ”plum pudding” model of the atom was a model for the structure of the atom where
an atom was made up of a large positively charged mass with small, negatively
charged electrons distributed throughout. In 1909, Ernest Rutherford performed an experiment that showed that the ”plum pudding”
model of the atom was incorrect. Describe Rutherford’s experiment and explain how it showed that the nuclear model of
the atom was correct.
Okay, so this question is talking about the ”plum pudding” model for the structure of
the atom. So, let’s start by drawing a diagram that shows the ”plum pudding” model so we know
what we’re talking about.
So, back in the day, this is what scientists expected atoms to look like. The big orange blob represents the large positively charged mass, and the little pink
dots are the negatively charged electrons that are distributed throughout. Now, we’ve been told in the question that in 1909 Ernest Rutherford performed an
experiment that showed that this model, the ”plum pudding” model, was incorrect. And what we’ve been asked to do are two things: firstly, we have to describe
Rutherford’s experiment and secondly, we have to explain how it showed that the
nuclear model of the atom was correct.
So what is the nuclear model of the atom? Well, it’s the model of the atom which contains a nucleus. So the nuclear model of the atom suggests that at the center of atom, there is a
small positively charged nucleus and all of the electrons orbit this nucleus. Now, Rutherford’s experiment showed that this model of the atom was incorrect, and
this model was correct.
So, let’s start by describing Rutherford’s experiment. What Rutherford did in his experiment was to take small positively charged particles,
known as 𝛼 particles, and he fired them at a thin sheet of gold foil. Now, this sheet of gold foil was indeed very thin. In fact, it was probably only a few atoms across in thickness.
The aim of this experiment was to see what happens to the 𝛼 particles as they pass
through the gold foil and interacted with it, specifically, as they interacted with
the gold atoms in the gold foil. This interaction between the 𝛼 particles and the atoms in the gold foil would give
some clues about the structure of the atom.
Now, if we zoom in a little bit, we can talk about what Rutherford expected to see
when the 𝛼 particles pass through the gold foil. Because he thought the gold foil was made up of ”plum pudding” atoms, he simply
expected the 𝛼 particles to pass straight through the gold foil because these 𝛼
particles would go straight through these ”plum pudding” atoms without interacting
very much with them.
And the reason for this is the following. Firstly, we need to remember that before Rutherford did his experiment, electrons had
already been discovered. This means that scientists already knew that there was some negative charges inside
atoms. However, scientists also knew that overall atoms were neutral. This means that they didn’t have a net positive or negative charge. They were neutral.
This meant that there must have been some positive charge in the atom to counteract
the negative charges of the electrons. However, if these positive charges in the atom were spread out over the entire atom,
like the scientist expected them to be, then their interaction with other positively
charged particles, such as 𝛼 particles, wouldn’t be as strong.
The reason for this is that in the ”plum pudding” model, all of the positive charges
were distributed over a large volume. Therefore, that effective strength was reduced, as compared with that same amount of
positive charge isolated in a very small volume because now in this situation you
still have the same amount of positive charge, but the charge density is much larger
cause you’ve shoved that same amount of positive charge into a much smaller
volume. So it interacts with other charged particles much more strongly in this
However, if the ”plum pudding” model was indeed true, then 𝛼 particles would just
pass straight through these atoms without interacting very much. And so what Rutherford expected to see was all of the 𝛼 particles passing straight
through the foil, maybe with some little deflection around a fraction of a degree or
so, but not much more than this.
However, this is not what Rutherford saw at all. Instead, what he saw was that some 𝛼 particles went straight through as expected,
some others were deflected by large amounts, and a very, very small proportion of
them were deflected so much that they actually came back towards the source.
This was the most shocking discovery of this experiment. In fact, when Rutherford saw that some of these 𝛼 particles were deflected so much
that they literally came back towards the source, he said that this was like firing
a 15-inch shell at a piece of tissue paper and the shell came back and hit you. And so this experiment suggested very strongly that we were dealing with a nuclear
atom, not a ”plum pudding” atom.
And the reason for this is the following. Firstly, in a nuclear model atom, most of the volume of the atom is actually empty
space. This means that a huge chunk of 𝛼 particles can just pass straight through if
they’re travelling far away from the nucleus. This accounts for the large number of 𝛼 particles that did indeed pass straight
But this was expected to be seen in the ”plum pudding” model as well. So what’s different? Well, in this model as 𝛼 particles get closer and closer to the nucleus, they
deflect more and more, because remember 𝛼 particles are positively charged and the
nucleus is also positively charged. And two positive charges always repel each other.
So this model accounts for the large number of 𝛼 particles that were actually
deflected by more than just a fraction of a degree. And finally, this model also explains some of the 𝛼 particles, the very small
fraction, that actually came back towards the source. Because only in this model, when an 𝛼 particle is on course to directly hit the
nucleus, will experience so much of a deflection that it actually gets sent back
towards the source.
This would not happen in the ”plum pudding” model because remember all of the
positive charge in the ”plum pudding” model is distributed over a large volume. So there’s not really an area of very, very strong positive charge that the 𝛼
particle can travel towards. Even if it went towards the center of the atom, it would experience a slight
deflection, whereas in the nuclear model there is a very densely packed region of
positive charge. And so the small number of 𝛼 particles travelling directly towards that will be
deflected straight back.
Hence, Rutherford’s experiment showed that the ”plum pudding” model of the atom was
indeed incorrect. And it provided some very solid evidence for the nuclear model to be correct. So at this point, we can go back to our question.
The last sentence of the question said describe Rutherford’s experiment and explain
how it showed that the nuclear model of the atom was correct.
So let’s start by describing Rutherford’s experiment. We can say that in the experiment, 𝛼 particles were directed at a thin sheet of gold
foil. And the prediction of the experiment was that if the ”plum pudding” model was
correct, then almost all the 𝛼 particles would have passed straight through and
almost none would have been deflected.
So this can be our description of Rutherford’s experiment, which means we need to
move on to explaining how it showed that the nuclear model of the atom was
For this explanation, we can say firstly that most 𝛼 particles did pass straight
through the foil as expected. However, some of the alpha particles, a very small proportion, were deflected through
very large angles. Remember the ones that came straight back at the source? Yeah! That’s these ones.
So what did this mean? Well, this meant that there must have been a very dense positive charge or in other
words a nucleus in the atom. Because remember in order for the 𝛼 particles to be deflected by that much, they
needed to be a very small region with all of that positive charge packed into
But, consequently, because there was a limited amount of positive charge in each atom
and all of that positive charge was packed into a very small volume, then that meant
that most of the atom was empty space. And thus, this showed that the nuclear model of the atom was correct. And this is our explanation for how Rutherford’s experiment showed that.