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DEVELOPING THE ATOMIC
THEORY
Text 5.1: Page170-175
Agenda
1.
2.
3.
4.
5.
Just How Small is an Atom?
Developing Atomic Theory
Modern Model
Check & Reflect
Assignment – Timeline of the Development of
Atomic Theory
Learning Goals

By the end of the class, we should be able to
 Describe
how the model of the atom has changed over
time
 Describe the structure of the atom according to modern
atomic theory
 Calculate the number of protons, neutrons and electrons
in an atom based on the periodic table
Minds On: Just How Small is an Atom?
Development of Atomic Theory

Philosophers of
Ancient Greece
reasoned there
were 4 basic
forms of matter
 Fire,
Water,
Earth and Air
Development of Atomic Theory

In ancient China
they reasoned there
were 5 basic forms
of matter
 Fire,
water, wood,
metal and earth
Development of Atomic Theory

440 BCE, Greek philosopher
Democritus also hypothesized
about the atom
 He
thought that breaking down
rock into powder would reduce
it to tiny bits of matter that
couldn’t be broken down any
more
 Wasn’t popular at time because
there was no experimental
evidence to support his claim
Modern Atomic Theory

Rooted in the idea that an
understanding of atoms and
their structure can help us
predict the properties of
matter
 Began
with John Dalton, who
reconsidered the ancient idea
that each different kind of
element is composed of a
different type of atom
Dalton’s Atom

Dalton’s theory stated:
All matter is made of small,
invisible particles called atoms
 Atoms of different elements
have different properties
 Atoms of different elements can
combine in specific ways to form
new substances
 All atoms of an element are
identical in proprieties such as
size and mass

Sound Familiar?
Building Off Dalton…

He had it right!
 All
matter is made of small,
invisible particles called atoms
 Atoms of different elements
have different properties
 Atoms of different elements
can combine in specific ways
to form new substances
 All atoms of an element are
identical in proprieties such as
size and mass
The Atom


JJ Thompson researched the idea that atoms may
be made of smaller parts that are charged
Experiment included using electric currents in glass
tubes called cathode ray tubes.
The Atom

JJ Thompson researched the
idea that atoms may be
made of smaller parts that
are charged
 Through
experimentation he
determined atoms must be
made of both + and –
charged pieces
 We now know these to be
electrons (-) and protons (+)
The Atom

Ernest Rutherford
expanded on the idea
of the size of the parts
of the atom
 He
calculated through
experimentation that
there was a tiny nucleus
in the center of orbiting
electrons
Finishing Touches

James Chadwick suggested in the
nucleus also contained neutral
particles


Now known as neutrons
Niels Bohr suggested electrons
traveled in shells or energy levels
With each shells having a specific
number of electrons in them
 Also now sometimes described as
“electron clouds”

So What’s the Big Picture?
So What’s the Big Picture?
More Info on the Atom

Note: Atomic Mass= # Protons + # Neutrons
Check & Reflect



Skim & Scan Chapter 5.1
Complete questions 1-6 on page 175 ‘Learning
Checkpoint’.
Answer in complete sentences in your notebook.
Assignment - TimeToast



Create a timeline of the development of the atomic
theory.
Include pictures, text, and info on two scientists per
time period.
Use p.g. 176 to help you…