HISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

he pounded up materials in his pestle and

mortar until he had reduced them to smaller

and smaller particles which could not be

divided and called these:

ATOMA

(greek for indivisible)

Historic Models of the Atom

• Aristotle (384-322 BC) didn’t think there

was a limit to the number of times matter

could be divided.

• He knew there were small particles. Air,

Fire, Earth, Water

HISTORY OF THE ATOM

1808 John Dalton

suggested that all matter was made up of

tiny spheres that were able to bounce around

with perfect elasticity and called them

ATOMS

Dalton’s Atomic Theory

1. All elements are composed of atoms (which

can’t be divided).

2. Atoms of the same element have the same

mass and atoms of different elements have

different masses.

3. Compounds contain atoms of more than one

element.

4. In a compound, atoms of different elements

always combine in the same way.

Dalton’s atom

Dalton’s Atomic Theory

Most of Dalton’s statements are now known to be flawed.

As we continue to study the atom, we will talk about discoveries that disproved Dalton’s statements.

Scientists have revised the theory due to new discoveries!

HISTORY OF THE ATOM

1898 Joseph John Thomson

found that atoms could sometimes eject a far

smaller negative particle which he called an

ELECTRON

J.J. Thomson, 1897• Discovered the electron, the subatomic particle

with a negative charge

• His experiments involved the use of a cathode

ray tube

Animation of

Thomson’s model

HISTORY OF THE ATOM

Thomson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere surrounded

by a soup of positive charge to balance the electron's charge

1904

like plums surrounded by pudding.

PLUM PUDDING

MODEL

Thomson’s Atomic Model

• Developed the Plum

Pudding model.

(think of a chocolate

chip cookie)

• Electrons evenly

distributed throughout

a positively charged

material.

Plum Pudding Model

e- e-

e-

e-

e-

Sphere of

Positive charge

HISTORY OF THE ATOM

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out his

famous experiment.

they fired Helium nuclei at a piece of gold foil

which was only a few atoms thick.

they found that although most of them

passed through. About 1 in 10,000 hit

something

HISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the helium nuclei passed

through the foil, a small number were deflected and, to their

surprise, some helium nuclei bounced straight back.

helium nuclei

Ernest Rutherford, 1911

• Tested theory that electrons were evenly

distributed throughout the atom within

positively charged material

• Performed the Gold-Foil Experiment

Rutherford animation

Another Rutherford Animation

Ernest Rutherford

• If Thomson’s model was correct, most of

the alpha particles should pass through

with a little deflection

Rutherford’s Model

• Proposed that atoms contain a nucleus, a

small, dense, positively-charged sphere in

the center of the atom.

• Atom contains mostly empty space.

• The nucleus is tiny compared with the

atom as a whole.

Ernest Rutherford

Thomson’s model Rutherford’s model

Rutherford’s model:

Nuclear Atom

e- e-

e-

+

+ +

Rutherford did

NOT know about

neutrons!

HISTORY OF THE ATOM

Rutherford’s new evidence allowed him to propose a more

detailed model with a central nucleus.

He suggested that the positive charge was all in a central

nucleus. With this holding the electrons in place by electrical

attraction

However, this was not the end of the story.

• The Houston

Astrodome occupies

more than nine acres

and seats 60,000

people.

• If the stadium were a

model for an atom, a

marble could

represent its nucleus!

HISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by adding

that the electrons were in orbits. Rather

like planets orbiting the sun. With each

orbit only able to contain a set number of

electrons.

Bohr’s Atom

electrons in orbits

nucleus

Summary

• Dalton’s Theory- all matter is made up of

atoms, which can’t be divided

• Thomson’s Model- discovered atoms were

made up of smaller particles (these

smaller particles are charged)

• Rutherford’s Theory- discovered the

positively charged nucleus