450 BC - Empedocles (Greece)

Four elements: Earth, Water, Air, Fire

experiments — tried to turn lead into gold

400 BC Democritus

Suggests “atomos”, meaning indivisible smallest

piece of matter.

​(Mixture of philosophers, mystics, magicians &

chemists) did experiments tried to turn lead into

gold!

1650 Many alchemists

“Element” – a pure substance that cannot be chemically broken down into simpler substances.

1700 Robert Boyle

Both isolated oxygen. Henry Cavendish proved

oxygen was in water.

1808 Joseph Priestley (England) & Antoine

Lavoisier (France)

• All matter made of tiny particles called atoms.

• Each element has its own kind of atom with its own mass.

• Atoms combine to form compounds.

• Atoms are not

created or destroyed by ordinary means.

1808 (AtomicTheory) -John Dalton’s Atomic

Model

1831 Michael Faraday

Found electrical currents could charge atoms —

called them ions. Matter contains positive and

negative charges. Opposites attract, likes repel.

Atoms form compounds due to electrical forces.

1897 J. J. Thomson

Discovered the electron.
Developed the Plum Pudding Model – a positive sphere with embedded

negative electrons..

1904 Hantaro Nagaoka

Proposed Saturn Model – a positive sphere with

negative rings (electrons orbiting like Saturn’s rings).

1911 Ernest Rutherford

Nuclear Model – small, dense, positive nucleus

surrounded by negative electrons.

1913 Niels Bohr

Planetary Model – nucleus with electrons in fixed

energy levels (like planets around the sun).

1932 James Chadwick

Discovered the neutron, a particle with no charge.