ATOMIC THEORYPart:1 Early Theories

Atoms

All matter is composed of tiny particles called atoms.

An atom is the smallest particle of an element that retains the characteristics of the element.

Evolution of the Atomic Model

Atoms are far too small to be seen - even with the most powerful electron microscopes!

Models have been developed to help us visualize what the atom looks like

Model: a verbal, mathematical or visual representation of a scientific structure or process

Early Days The Four Element Theory: (lasted for about

2000 years!)

According to Greek philosopher Aristotle, all matter was comprised of four elements: Earth, water, air and fire.

Early days still… First mention of atoms!!!

In 300 BC, Democritus, a Greek philosopher said atoms were indivisible particles.Smart guy.

Dalton’s Model of the Atom

In early 1800’s, John Dalton ran an electric current through waterElectrolysis Hydrogen (H2) and oxygen (O2) bubbles were

formed

They have very different physical and chemical properties so their particles must be different.

The volume of hydrogen was twice that of oxygen.

Dalton’s Model of the Atom

With this (and other information), he formed the famous…

Dalton’s Atomic Theory…

I have a theory…

Dalton’s Atomic Theory

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

2. Atoms cannot be created or destroyed, or divided into smaller particles.

3. All atoms of the same element are identical in mass and size. The atoms of one element are different in mass and size from the atoms of other elements.

4. Compounds are created when atoms of different elements link together in specific proportions.1. H2O

2. CO2

Dalton’s Model of the Atom This was a huge development in

science!

This model was excellent, but more research showed that it required amendments.

The Next Big Discovery Following Dalton’s electrolysis work, other

scientists studied gasesThey applied electric current to different gases in a

tube and observed how the gases glowed

What is a gas discharge tube?

A gas-discharge tube uses electrodes inside a low-pressure gas to create light. When a high potential difference exists between the electrodes, the gas forms a plasma. Free electrons flow between the electrodes.

http://www.youtube.com/watch?v=GzMh4q-2HjM

Since the cathode rays were attracted to the positive plate, the particles that make up cathode rays must be composed of negative charges.

The Electron

J.J. Thompson used the gas discharge tube to gather evidence of the existence of these negative particles

These negative particles were named electrons (1987)

The Electron

After experimenting with many different materials, he was surprised to see the same ray produced each time.

These electrons were found in ALL atoms.

Electrons were the first subatomic particles to be found.

New Atomic Model J.J. Thomson revised Dalton’s atomic model to

reflect the newly discovered electrons

He called it the “Raisin Bun Model” Atom have electrons embedded within, like raisins in

a bun….

Raisin Bun Model

Thomson knew the atoms were electrically neutral Positive charge = negative charge

These discoveries earned him the Nobel Prize in 1906

Soon enough, one of Thomson’s students proved him wrong!