Development of the atom

Who made the first theory?

The ancient Greeks : 500 BC

What was the first theory?

That everything is made up of tiny particles that can’t be broken down any further

Who made the second theory?

John Dalton : 1800’s

“Solid Spheres”

What was the second theory?

That everything is made up of solid spheres and different types of spheres make up different types of elements

Who made the third theory?

JJ Thompson : 1897

“Plum Pudding Model”

What was the third theory?

Atoms must contain negatively charged particles.

The atom is a ball of positive charge with negative electrons embedded into it.

Who made the fourth theory?

Ernest Rutherford : 1909

“Nuclear Model”

What was the fourth theory?

Atoms have a compact nucleus of positive charge and the negative charge exists in a cloud around it.

Atoms must be mainly empty space.

How was the fourth theory discovered?

The gold foil experiment

What was the method of the gold foil experiment?

Alpha particles were fired at a very thin piece of gold foil (only a few atoms thick)

What were the results of the gold foil experiment?

• Most of the atoms passed straight through the foil

• Some were deflected but still passed through

• Some were reflected

What did most of the alpha particles passing straight through mean?

That atoms were mainly made of empty space

What did some of the alpha particles being deflected mean?

That atoms must have a centre of positive charge that occasionally came into contact with the positive alpha particles and therefore repelled them

What did some of the alpha particles being reflected mean?

That atoms must contain a great deal of mass at the centre

Who made the fifth theory?

Neils Bohr : 1913

What was the fifth theory?

That electrons are held in shells around the positive nucleus and orbit it like how planets orbit the sun

Who made the sixth theory?

Ernest Rutherford

What was the sixth theory?

That the nucleus is made of small discrete particles that we now know as protons

Who made the seventh theory?

James Chadwick

What was the seventh theory?

That the nucleus also contains neutral particles that have no charge, which we now know as neutrons