Download Atomic Theory Part 1 - Early Theories

Part: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 water
 Electrolysis
 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…
Dalton’s Atomic Theory
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.
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 gases
 They applied electric current to different gases in a
tube and observed how the gases glowed
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.
What is a gas discharge tube?
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!