Atomic Theory

Greeks

Matter is made of tiny indivisible particles

John Dalton

1. Atoms are indivisible and indestructible

2. Atoms of a given element are identical

3. Atoms of different elements vary in mass

4. A compound contains atoms of two or more elements combined in a fixed ratio

Law of conservation of mass

Matter cannot be created or destroyed only changed from one form to another

What is this?

Discharge Tube

William Crookes

Modified the discharge tube by adding a free moving paddle wheel and Maltese cross.

The wheel moved towards the anode meaning the cathode rays are particles.

The Maltese cross cast a shadow on the anode meaning the cathode rays travel in straight lines.

J.J. Thomson

Modified the discharge tube by adding two electrodes above and below the tube.

The cathode rays were attracted to the anode meaning that the cathode rays are negatively charged.

Negative particles are lighter then H atoms meaning they are sub-atomic.

Measured the charge to mass e/m

George Stoney

Named the sub-atomic particles electrons

Robert Millikan

1. Oil is vaporised into droplets and falls into viewing chamber (with charged plates above and below) under gravity

2. X-rays ionise the air in the chamber and droplets take on a negative charge

3. The droplets are then balanced by varying the electric field and results were fed into e/m - electrons mass was calculated

What is this?

Thomson’s Plum Pudding Model

Ernest Rutherford - Observations

1. Fired positive alpha-particles at gold foil and most went through

2. Some slightly deflected and a few deflected by huge angles

3. 1 in every 100,000 completely bounced back towards their source

Ernest Rutherford - Conclusion

1. An atom is mostly empty space

2. There is a region of strong positive charge in the atom

3. There is a solid mass at the centre of the atom

What is this?

Rutherfords Model of the Atom (new model of the atom)

James Chadwick

Discovered neutrons by knocking them out of beryllium using alpha-particles and identified them as neutral particles in the nucleus