Democritus: Believed all matter consisted of extremely small particles that could not be divided.

Called these particles “Atomos”- Greek word meaning uncut or indivisible

Ancient Greek Models

Aristotle: Believed there was no limit to the number of times matter could be divided

Believed all substances were built from only four substances: earth, air, fire, and water.

Ancient Greek Models

Remember, this was all just speculation!!!

Through experimentation, Dalton discovered that elements in a compound always join the same way.

He proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. His theory is as follows:

1. All elements are composed of atoms2. All atoms of the same element have the same

mass3. Compounds contain atoms of more than one

element4. In a compound atoms of different elements

combine the same way.

Dalton’s Atomic Theory

Dalton’s theory explained data from many experiments for a while, but over time scientists found all of Dalton’s ideas about atoms were not correct. They did not discard his theory, but they revised it.

A Theory Revised

John Dalton (born 1766)

Investigations in the 1800’s proved that atoms were in fact divisible.

ATOM- smallest particle of an element that retains the chemical properties of that element.

HISTORY OF THE ATOM

Thomson’s Model of the Atom

1898 Joseph John Thomson

• Thompson’s experiments provided the

first evidence that atoms are made of

even smaller particles.

•Experiments using electric currents

were carried out in tubes know as

cathode-ray tubes.

Cathode Rays and Electrons (Thomson’s Experiment)

Cathode-Ray Experiment

• Observed that cathode rays were deflected by a magnetic field (like wire carrying electric current) which had a negative charge.

•Rays were also deflected away from a negatively charged object and attracted to the positive plate inside the tube.

•CONCLUSION: Cathode Rays are negatively charged!

Thomson- found evidence that there were negative charges scattered throughout an atom!

HISTORY OF THE ATOM

Thompson developed the idea that an atom was made up of

“electrons,” which are negatively charged. They are

scattered unevenly within an sphere surrounded by a soup

of positive charge to balance the electron's charge like

plums surrounded by

pudding.

1904

PLUM PUDDING

MODEL

Negative charges scattered throughout an atom filled with a positively charged mass.

HISTORY OF THE ATOM

1910 Ernest Rutherford

Disproved the PLUM PUDDING model

Rutherford and his team fired alpha

particles at a piece of gold foil which was

very thin.

This experiment led to the discovery of a

Nucleus, where all of the atoms positive

charge is concentrated.

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

HISTORY OF THE ATOM

Rutherford’s new evidence allowed him to

propose a more detailed model with a central

nucleus that was densely packed.

He suggested that the positive charge was

all in a central nucleus, holding the electrons

in place by electrical attraction

However, this was not the end of the story.

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, moving in many directions, like

planets orbiting the sun, with each

orbit only able to contain a set number

of electrons.

Bohr’s Atom

electrons in orbits

nucleus

Each electron in an atom has a specific amount of energy, and an atom can gain or lose energy.

The possible energies that electrons in an atom can have are called energy levels.

An electron cannot exist between energy levels. An electron in an atom can move from one

energy level to another when the atom gains or loses energy.

**Bohr was correct in assigning energy levels to electrons, but incorrect in assuming that electrons moved like planets in a solar system. We will discuss this further later on.

Bohr’s Model

HELIUM ATOM (according to Bohr’s Model)

+N

N

+-

-

proton

electron

neutron

Shell

Proton has positive charge equal in magnitude of negative charge of electron= neutral atoms

nucleus