Download Atomic Theory

The Development of
Atomic Theory
I. Early Models of
Atomic Structure
The work of Dalton,
Thomson, and Rutherford…
Democritus,
"The Laughing Philosopher"

Democritus was a preSocratic philosopher who
said that all matter is
made up of various
indivisible elements
which he called atoma,
from which we get the
English word atom.
John Dalton’s
Model of the Atom


John Dalton
developed his
atomic theory
in 1803
It worked well,
until subatomic
particles were
discovered.
Dalton’s atomic theory
1.
2.
3.
4.
All matter is made of atoms, which are
indivisible and indestructible particles.
All atoms of an element are all identical
in mass and properties.
Atoms of different elements have
different masses and properties.
Compounds are formed by atoms
combining in small whole number ratios.
J.J. Thomson’s Model of the Atom


J.J. Thomson
discovered the
electron while
studying
cathode ray
tubes in 1897.
He received the
Nobel Prize in
1906.
Thomson’s cathode ray tube
(a vacuum tube with 2 electrodes)
His discovery of the electron…

Thomson found that the cathode ray was
a beam of negative particles (electrons)
and so atoms were NOT indivisible.
Thomson’s “Plum Pudding” Model
of the Atom

He described his atomic
model as negative
charged electrons
scattered in a lump of
positively charged
material, like raisins
scattered in plum pudding
(a popular dessert at the
time).
Rutherford’s Model of the Atom


In 1894, Ernest Rutherford was
awarded a scholarship to be a
research student at the
Cavendish Laboratory under
J.J. Thomson. He received the
Nobel prize in 1908.
Rutherford’s Gold Foil
experiment led to the
development of his atomic
model in 1911.
The Gold Foil Experiment

Rutherford fired a
beam of positively
charged particles
(called alpha
particles) at a
sheet of gold foil a
few particles thick.
The Gold Foil Experiment

Rutherford was
expecting results
in line with
Thomson's
model, with the
stream of positive
particles passing
through the foil.
The Gold Foil Experiment

Instead, he
observed that
some of the
alpha particles
were repelled,
while most went
through the foil
unchanged.
The Gold Foil Experiment
There was only one explanation…


A dense, very positive
charge was condensed
into one place, called
the nucleus.
The rest of the atom had to be made up of
mostly empty space. He described the
electrons as “buzzing around the nucleus like
bees buzz around a hive”.
Rutherford’s Model of the Atom

One particularly memorable quote attributed to
Rutherford is "All science is either physics or
stamp collecting“.
II. Understanding the
basics…

Let’s look at some basic ideas of physics to
better understand the more recent models of
the atom. We need to understand wave
properties, and light energy.
Wave Properties
–Wavelength, λ, is the distance
between two like points on a wave
[the unit of wavelength is the meter (m)]
Wave Properties
–Frequency, υ, describes the number
of wave cycles per second.
[The unit of frequency is cycles/second (s-1),
or the Hertz (Hz)]
Wave Properties

Amplitude is the maximum height
of a wave, measured from the
origin line of the wave.
[A wave has zero amplitude at certain intervals
along the wave, called nodes.]
The Electromagnetic Spectrum
Short wavelength…………………….long wavelength
High frequency………………….………..low frequency
High energy……………….…………………..low energy
The Electromagnetic Spectrum
 All
electromagnetic radiation
(including visible light) travels at
the same speed.
 The
speed of light (c)
= 3.0 x 108 m/s
Questions:



Which has the highest frequency, red light
or green light?
Which has the longest wavelength, x-rays
or microwaves?
Which has the highest energy, yellow light
or infrared rays?
The wavelength and frequency of
light are inversely related.
C = lu
Speed of light = wavelength x
frequency
(Since c = 3.0 x 108 m/s, you will be asked
to solve for wavelength or frequency.)
l = c/u
u
Question:


Orange light has a wavelength of
620 nm. What is the wavelength in
meters? What is the frequency?
620 nm (10-9 m/1 nm) = 6.2 x 10-7 m
u = c/ l


u
m
=
3.0 x 10 m/s / 6.2 x 10-7
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