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Lesson 5
Atomic Theory
Anything in black letters = write it in
your notes (‘knowts’)
Section 1 – Defining the Atom
Section 2 – The Spectrum of Light
Section 3 – Development of Modern Atomic Theory
Section 4 – The Nucleus
Section 5 – Electrons in Atoms
How do we know that atoms exist?
5.1 – Defining the Atom
Democritus (460 B.C. – 370 B.C.)
one of the first to propose the
idea of the atom; based on
pure speculation
comes from the Greek word atomos
which means uncuttable or indivisible
John Dalton (~1800)
proposed 1st atomic theory
Daltons Atomic Theory (~1800)
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
3. Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form compounds.
4.
Chemical reactions occur when atoms are separated from each other, joined,
or rearranged in different combinations. Atoms of one element are never
changed into atoms of another element as a result of a chemical reaction.
Atoms of
element A
Atoms of
element B
Mixture of atoms of
elements A and B
Compound made by
atoms of elements A
and B
What we know now of Dalton’s Atomic Theory
1. All elements are composed of tiny indivisible particles called atoms.
Atoms are not indivisible – they are made of subatomic particles
2. Atoms of the same element are identical. The atoms of any one
element are different from those of any other element.
Every atom has at least one isotope; one atom’s isotope is NOT
identical to another isotope of the same atom.
3. Atoms of different elements can physically mix together or can
chemically combine in simple whole-number ratios to form compounds.
This is known as the Law of Definite Proportions – very important.
4.
Chemical reactions occur when atoms are separated from each other, joined,
or rearranged in different combinations. Atoms of one element are never
changed into atoms of another element as a result of a chemical reaction.
Atoms of one element can change into an atom of another element
as a result of a nuclear reaction.
Law of Definite Proportions Atoms combine in fixed whole number ratios.
CO2
H 2O
Law of Multiple Proportions Sometimes, atoms combine in more than
one ratio.
CO, CO2
H2O, H2O2
Atom - smallest particle of an element that still
has the properties of the element
5.2 – The Spectrum of Light
Light waves can be described by their
wavelength or frequency.
Wavelength ()– length of the wave!
 - “lambda”
measured in meters
Frequency ()– number of waves per second.
 - “nu”
measured in Hertz (Hz) or waves/second
(how frequent the waves are)
In a vacuum, light travels at 3.00 x 108 m/s
c  
speed of light
(m/s)
wavelength
(m)
frequency
(1/sec)
The Electromagnetic Spectrum
Low energy
( = 700 nm)
Frequency  (s-1)
3 x 106
102
High energy
( = 380 nm)
3 x 1012
3 x 1022
10-8
10-14
Visible light ranges from 380 – 700 nm
1 nm = 10-9 m
Example: Calculate the frequency of light with a
wavelength of 532 nm
First, make sure the wavelength is in meters (not nm).
532 nm = 532 x 10-9 m
c= 
3.00 x 108 = (532 x 10-9)
0.00564 x 1017 Hz= 
5.64 x 1014 Hz = 
Light behaves as a wave but consists of
particles called photons.
Photon - particle of light
Higher frequency photons have a higher energy.
E = h
h = Plank’s constant = 6.626 x 10-34 J·s
Example: Calculate the energy of red light with
a wavelength of 700 nm
To calculate the energy, we need to know the frequency.
All light waves travel at c, the speed of light.
c=
3.00 x 108 = (700 x 10-9)
4.2857 x 1014 Hz = 
E = h
E = (6.626 x 10-34)(4.2857 x 1014)
E = 2.84 x 10-34 J
c  
Eh
E  h
c

This equation may save some time…
Incandescence – emission of light by hot objects.
Incandescent light bulbs give off more heat energy than
light energy and so are not very efficient.
Fluorescence – absorption of light of high
freq. and re-emit at lower freq.
Blacklight – emits UV light and small amount
of visible light.
A fluorescent material will absorb the high energy UV light
and re-emit lower energy visible light – the material then
appears to “glow” in the dark.
Fluorescent Lights
When Hg is energized, it produces UV light. The white
powder in a fluorescent light bulb will fluoresce and visible
white light is emitted.
VIDEO – half-coated fluorescent tube
Phosphorescence – absorption of light
followed by a delay in the re-emission of light.
Phosphorescent materials will keep emitting
light for a while in the dark.
Phosphorescent pigments
In visible light
ZnS
SrAl2O4
Phosphorescent pigments
In the dark
ZnS
SrAl2O4
Phosphorescent pigments
In the dark after 4 minutes
ZnS
SrAl2O4
Chemiluminescence – emission of light by
a chemical reaction.