1. Atomic Theory

2. Democritus
400 BCE, Greek Philosopher Democritus – developed theory that if you continuously cut an object, you would eventually end up with a particle that could not be cut.
He called the particle an atom – from the Greek work atomos meaning not able to be divided.

3. Dalton Published his atomic theory in 1803:
All substances are made of atoms that cannot be created, divided, or destroyed
Atoms of the same element are exactly alike
Atoms join with other atoms to make new substances

4. Thomson Discovered that there are small particles inside the atom
Experimented with a cathode-ray tube, which had charged metal plates. He found the ray bent toward the positive plate, which proved that there are negatively charged particles present in an atom – called electrons
Designed the “plum-pudding” model, which placed electrons mixed in with the solid atom

5. Thomson’s Plum Pudding Model

6. Rutherford
Aimed a beam of small, positively charged particles at a thin sheet of gold foil.
He discovered that atoms are made up of mostly empty space, with a solid, positively charged nucleus.

7. Bohr 1913 – Studied the way atoms react to light.
He proposed that electrons move around the nucleus in certain paths, or energy levels.
He believed electrons could jump between levels, but must follow a path in the level.

8. James Chadwick In 1932, James Chadwick discovered the neutron.
In his experiments, he discovered that the atomic mass of an atom was double what it should be based on the number of protons.
However, the overall charge of the atom was not affected by this extra mass, leading him to discover the neutron which has a neutral charge.

9. Electron Cloud
The modern atomic theory – electrons do not travel in definite paths
The exact path of an electron cannot be predicted, but there are regions inside the atoms where the electrons are likely to be found. This region is called the “electron cloud”