1. The History of the Atom

2. What is matter made of?
This is a difficult question, because we can’t see the smallest parts of matter

3. Empedocle A Greek around 400 BC
He divided matter into four elements, that he also called "roots":
These roots were
Earth
Air
Water
Fire

4. Democritus (another Greek)
Named the smallest piece of matter the atom which means ‘indivisible’
He and his followers thought that all atoms were made of the same material but were different shapes and sizes
His ideas were forgotten for 2100 years

5. But these were just ideas
In the 1800’s a man named Dalton was investigating the properties of gases
He noticed that gases combine as if they were made of individual particles
His theory of atomic structure is called the Billiard Ball Model

6. The Billiard Ball Model
Atoms are indivisible and indestructible
Atoms of different elements are different
Chemical compounds are formed by combining atoms

7. Remember, Dalton had never seen an atom!

8. Thomson In 1897 he came up with a different model
He did experiments that showed there were negatively charged particles in atoms
He called these particles electrons

9. Raisin Bun Model
Thomson thought the atom was made of positively charged material with negatively charged particles embedded within it
Sometimes this is called the ‘Plum Pudding Model’

11. Thomson had never seen an atom!
Thomson was doing experiments where he passed an electric current through gases
He discovered that negative particles were being emitted from the gas
Therefore atoms must contain charged particles
Thomson’s model was based on his experimental evidence
His model explains the evidence that he collected.

12. Rutherford
In 1908 this English scientist was shooting alpha particles at very thin pieces of gold foil
Alpha particles are heavy particles with a positive charge
The gold foil was very thin

13. What Happened?
He expected that the alpha particles would pass through the gold foil without a problem
A screen behind the gold foil was used to detect the alpha particles after they passed through the foil

14. This is his experiment

15. His results were surprising!
Instead of passing through the gold foil, some of the particles bounced straight back in the opposite direction
Most of the particles passed right through
What did this mean?

16. Can you imagine?
Rutherford said it was like shooting a cannonball at a piece of tissue paper and having it bounce back

17. Rutherford explained his results by coming up with a new model of the atom
The atom must be mostly made up of empty space
That is why most of the alpha particles passed through the gold foil

18. What was Rutherford’s explanation?
There must be a very dense, positively charged region in the centre of the atom
That is what caused the alpha particles to change direction
Rutherford called this the nucleus

20. Atoms are electrically neutral
Therefore there must be negative particles in an atom
Rutherford thought that they orbited around the nucleus
He called them electrons

21. Rutherford’s Model is similar to our picture of the Solar System
Sometimes it is called the Planetary Model

22. Think of a marble in a baseball stadium
That is the size of the nucleus compared to the size of an atom!

23. In 1913 Rutherford’s model was revised by Neils Bohr
Bohr noticed that only certain energies of light would interact with the electrons of atoms
This meant that electrons could only have certain energies

24. Bohr’s Model
His picture of the atom had electrons in specific orbits around the nucleus
These orbits had certain energies

26. Bohr’s Model of the atoms is very useful even today
Later experiments have shown that the atom is really very complicated
The motion of particles can only be described with complicated math
Quantum Mechanics uses math to predict the movement of electrons
Bohr’s Model is still used for simple descriptions of atoms

27. The End!!!