Download Learning session 2: Models of atomic structure

Learning session 2: Models of atomic structure
[Referring to models on display from previous session] Those are all excellent model ideas, and they are a
lot like the ideas scientists were having about atomic structure from about 1890 to 1925. A man named JJ
Thompson, who discovered the electron, came up with a model that had electrons swimming around inside
a soup that had a positive charge. Protons and neutrons hadn't been discovered then, so that was as far as
he could go. Thompson was English and his model resembled a certain type of dessert that was popular in
England at the time so it became known as the "Plum Pudding" model.
Let's watch a clip about JJ Thompson and his work at the famous Cavendish Laboratory at Cambridge
University.
Video presentation
See The Discovery of the Electron
Things changed a great deal in 1911 because of the work of a very famous New Zealander, Ernest
Rutherford. He had already received the highest award for science in the world, the Nobel Prize, in 1908 for
showing that the element radium naturally breaks down into different elements but what he discovered next
was even more important. When radium does break down, a very small positively charged part of it flies off
at very high speed with a lot of energy. Ern (as he liked to be called) knew that there was a positive charge
inside the atom, and he knew that it should repel these positively charged bullets. He suspected that if he
fired a stream of these at a target made of thin gold foil they would be deflected from a straight line path by
whatever had the positive charge in the metal. Now because there are some of these "bullets" flying around
naturally, he wanted to make sure that what he was seeing were the ones he was firing, so he put one of
his detectors somewhere where it should only see the normal ones and not the ones he was firing, and that
was on the near side of the foil. That way he would be able to subtract those from the results he got. When
he started his experiment he got a big shock. A lot of the bullets were bouncing back almost in the direction
he'd fired them from!
He said soon after "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."
What Rutherford deduced from this was that the positive charge contained in each atom, and almost all of
the atoms mass, must be within just a tiny tiny part of it, which he called "the nucleus". He had discovered
that the atom was almost entirely empty space!
That changed the way people looked at atomic structure and the new "Rutherford model" was developed,
with a central nucleus and electrons a long way outside this.
This diagram shows a modified Rutherford model, incorporating protons and neutrons, both discovered after
its proposal. Symbols such as this are still commonly seen, although this model has since been updated to
support new findings.
Rutherford went on to discover the proton, and one of his students, James Chadwick, discovered the
neutron, and he got the Nobel Prize for that!
Let's watch this fascinating documentary to hear their incredible story.
learning session 2 - models of atomic structure.docx | Page 1
Video presentation
See ATOM
Scientists knew that there had to be more to the way electrons were placed though, and the next advance
came from a man named Niels Bohr. He came up with the idea of electron "shells", like the rings of an
onion, with electrons only being able to exist within a shell, and never in the space between them, and each
shell only being able to hold a certain maximum number of electrons at a time, depending on how far it was
from the nucleus.
This also neatly accounted for why electrons, with a negative charge, were held quite strongly within the
atom by the positive charge in the nucleus, but weren't completely sucked into it. They couldn't be because
there was no shell there in the nucleus for them to move to.
The idea that energy came in packets proved to be very important, and the packets were even given their
own name: "quanta", from the Latin word "quantum", meaning "how much". We see this in other words like
"quantity", and even "count".
Activity choices
Choose one of the following activity to undertake:

Create models that represent the historical development of the understanding of the atom and its
particles

Research and share your findings on one or more of these scientists; Ernest Rutherford; J. J. Thompson;
James Chadwick; Niels Bohr; Max Planck.

Watch one of the following clips and report back to the class what you have learned

>
Bill Nye the Science Guy- Atoms (1/2)
>
Bill Nye the Science Guy- Atoms (2/2)
>
What are atoms?
>
Atoms, from Chem4kids
Have a go at this online quiz about atoms
learning session 2 - models of atomic structure.docx | Page 2