History of the atom

Democritus

(400BC) Greek philosopher predicted that all matter is made of tiny particles called "atomos" or atoms

John Dalton

1808. Carried out many experiments on gases

John Dalton's Atomic Theory

1) All matter is made up of small particles called atoms

2) All atoms are indivisible. They cannot be broken down into simpler particles.

William Crookes

(1875) discovered cathode rays

First experiment (WC)

He passed an electric current through vacuum tubes.

He found that there was a sharp shadow of a Maltese cross at end of tube and that the glass was seen to fluoresce.

First experiment conclusion (WC)

Cathode rays travel in straight lines and have sufficient energy to cause glass to fluoresce when they strike it.

Second experiment (WC)

To investigate properties of cathode rays. In a vacuum tube he placed a light paddle next to a negative cathode. He observed:

that the paddle rotated and traveled down the vacuum tube

The vanes also rotated away from the negative cathode and towards the anode.

Second experiment conclusion (WC)

Cathode rays contain particles and have enough energy to move a paddle wheel.

JJ Thomson

(1897) discovered the electron

First experiment (JJT)

He passed a narrow beam of cathode rays through a small hole in the anode. The beam passed through two parallel plates and struck a zinc sulphide screen and made it glow.

When a positive charge was placed on the top plate, the beam of cathode rays attracted towards the top plate.

First experiment conclusion (JJT)

Cathode rays consist of negatively charged ions

Second experiment (JJT)

Outside the vacuum he placed a large electro magnet. Switching on the magnet he observed:

Electrons also deflected in this new magnetic field

Adjusting the strength of the magnetic field he brought the beam back down to original position.

Results (JJT)

Ratio of charge (e) of the electron to is mass (m). Abbreviate to e/m. Its value = 1.76 x 10^11 C/Kg. He couldn't work out the individual values.

Cathode rays

Are negatively charged particles called electrons. They travel in straight lines from cathode to anode. Are deflected by electro and magnetic field and have sufficient energy to move a small object.

Robert Millikan

1909 oil drop experiment, to determine the value of e (charge of electron)

Plum Pudding Model (JJ Thomson)

An atom is a sphere of positive charge and negatively charged electrons are embedded in the sphere at random

Who discovered the nucleus?

Ernest Rutherford, Hans Geiger, Ernest Marsden

Ernest Rutherford

(1909)He studied scattering of charged alpha particles by a thin sheet of gold

Observations of Gold Foil Experiment

1. Most particles passed straight through undeflected

2. Some alpha particles are deflected at large angles

3. A tiny number of alpha particles were deflected back along their original path (180°). Head on with nucleus of atom

conclusions of gold foil experiment

1. The atoms is mostly empty space

2. Alpha particles are repelled when they pass near the small, positive nucleus

3. The alpha particles collide head on with the dense positive nucleus

Discovery of the proton experiment

A radioactive source released alpha particles into a metal tube and collide with Nitrogen a totally inert gas. The high impact collision of the alpha particles in the nucleus results in proton repulsion. The ejected proton collided with silver foil screen and produces flashes.

Mass of proton, neutron, electron

Proton = 1

Neutron = 1

Electron = 1/1838

Discovery of neutron

James Chadwick 1932

James Chadwick experiment

bombarded Beryllium with alpha particles. He detected radiation coming from be-4. These neutral particles had energy to knock electrons out of paraffin wax. He proposed alpha particles knocked neutral out of beryllium