1. The Structure of the Atom
AQA Syllabus A
A Level Physics – Module 2
© T Harrison. The National School

2. The Very Early Days of Atoms
As far back as the 5th century B.C. the Greek philosophers Democritus and Leucippus proposed that matter was made up of tiny, indivisible particles in constant motion.
‘Atom’ comes from the greek word ‘atomos’ meaning ‘uncuttable’.
So, even thousands of years ago there was the idea that there must be very small objects that make up everything.

3. The Next Step for Atoms
Democritus had a problem with his idea of atoms – he had no proof. It was all just thoughts and because of this it wasn’t until the early 1800s that atoms came back onto the scene.
John Dalton proposed the Atomic Theory in 1803, and from his experiments and observations, he suggested that atoms were like tiny, hard balls.
He also inferred that each element has it’s own type of atom which combine together in certain ratios in compounds.
We now have a type of atom for each element, and evidence that supports their existence.

4. A Spanner in the Works
Everything had been going fine for atoms, until in 1896 A.H. Becquerel discovered penetrating radiation – how could anything get through solid atoms?
Then, in 1897 J.J. Thompson discovered the electron.
- He found that they had a negative charge.
- He also found that it would take about 2000 electrons to weigh the same as the lightest atom, hydrogen.
So, the smallest, ‘unsplitable’ items of matter are made of smaller pieces .

5. Thompson’s Experiments
Thompson carried out a number of experiments to identify what cathode rays consisted of.
He used magnetic fields to deflect them and found that the negative charge could not be separated from the cathode rays.
He then used the equipment below to show that the cathode rays were deflected by an electric field. Once he could measure how big the deflection was he would know the ratio of the charge to the mass (e/me).

6. The ‘Plum Pudding’ Model
Thompson then made a model including these electrons. It had them embedded in a cloud of positively charged gas (atoms themselves carry no overall charge, so the charges must balance out).
It was called the ‘Plum Pudding’ model, and could allow for electrons and for radiation to pass through. It could also explain electric currents flowing.
Electron
Positively charged gas

7. Another Change of Ideas
In 1911 Ernest Rutherford conducted an experiment to fire a beam of alpha particles through a very thin gold foil. He wanted to look at how the foil would scatter (deflect) the beam.
If the Thompson (Plum Pudding) model was correct, the beam would be very unlikely to scatter through an angle larger than a fraction of a degree .

8. Rutherford’s Experiment

9. Rutherford’s Model
Rutherford came up with a model of the atom that had a small, dense nucleus at the centre around which was a space where the electrons orbit.
In 1919 Rutherford finally separated, and discovered, the proton. Then another problem came up – how did they stick together?
In 1920 Rutherford speculated the existence of a neutral particle in the nucleus that helped to stick it together. It wasn’t until 1932 though that James Chadwick discovered the neutron.

10. Still a Problem
A problem still remained with Rutherford’s model. An orbiting object ought to lose energy with each change in direction making it spiral inwards.
In 1912 a Danish Physicist, Niels Bohr, came up with a theory that said the electrons do not spiral into the nucleus and came up with some rules for what does happen.
Bohr said, "Here's some rules that seem impossible, but they describe the way atoms operate, so let's pretend they're correct and use them."

11. Bohr’s Atom
RULE 1: Electrons can orbit only at certain allowed distances from the nucleus.
RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.

12. Bohr’s Atom
The Bohr Atom

13. More Complications
Bohr’s original model was fine for Hydrogen but did not work so well with heavier atoms.
From here on things got more and more complicated with ideas from Pauli, Heisenberg and others. While the mathematical concept of the atom got better, the visual concept got worse.
The world of the atom began to appear very strange. It proved difficult to form an accurate picture of an atom because nothing in our world really compares with it.
Bohr’s model (with the electron shells) is essentially what we use today, however, as it is a very useful visual tool.

14. The End