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The Scientific Method
Scientific Method
A series of logical steps to follow,
including experimentation, in order to
solve problems or answer questions.
The Process is NOT linear but circular.
Observe
Question
Reflect/generate
new questions
Repeat expt.
Results partially
Revise hypothesis
support hypothesis
OR
Results don’t
Hypothesis
Design/Conduct
Experiment
Evaluate Data
support hypothesis
Results support
hypothesis
.
Publish results
Repeat
experiment
Hypothesis
Preceded by a question
Takes the general form:
If “question” , then “prediction” or “outcome”,
because “tentative explanation”.
Example: QUESTION: why doesn’t my flashlight
light?
HYPOTHESIS: If a flashlight doesn’t light then
the batteries are exhausted, because the power
for the flashlight comes from the batteries.
A scientific hypothesis MUST be testable!!
THEORYS – Explain
• In Science, a Hypothesis is an attempt at an
explanation for the events that have been
observed. A hypothesis has to be testable.
• If a lot of evidence (data) is collected
through experiments to support the
hypothesis, then scientists accept the
hypothesis as a good explanation.
• An accepted hypothesis may, given even
more testing and agreement, become an
accepted theory. Usually takes many years.
LAWS – Describe
• Scientific Laws DESCRIBE observations,
they DO NOT EXPLAIN them.
• Experiments are not usually associated with
Laws.
Remember this very important idea: Isaac
Newton knew what gravity did, but he could
not explain why gravity did it. Even today the
topic of “what is gravity” is an active topic
for scientific discussion using Einstein’s
general theory of relativity.
You Can Never Turn A
Theory Into A Law!!
Summary
• Scientific Method is Circular
• Science is Social (scientists talk
to scientists)
• A Law Describes
• A Theory explains
• Can’t change a law into a
theory or vice versa.
The Modern Atomic
Model of the Atom
Dalton, Thomson and
Rutherford
What do We Know
About the Atom?
and…
How do we know what we know?
Outline
Early ideas about the atom
Discovery of the electron
Discovery of the nucleus and protons
Atomic Number, Mass Number
Atomic Mass
Greek Model (Democrites)
Everything is composed of earth, fire, wind
and water
Atoms are the smallest building block of
matter
Atoms are indivisible
All atoms are made of the same thing.
Atoms of different materials are different
shapes. (tree atoms look like little trees, rock atoms
look like little rocks)
Dalton’s View of the Atom
(1803)
Elements are made of tiny indivisible particles called atoms.
All atoms of one element are unique from atoms of other
elements
All atoms of a given element are identical and have the
same properties.
Atoms of one element can combine with atoms of other
elements in whole number ratios to form compounds. (Law
of constant composition)
Atoms are not created or destroyed in a chemical reaction
they are just rearranged into new compounds. (Law of
conservation of matter)
Crooke’s Tube
Cathode ray
Cathode
(-)
Anode
(+)
MAGNET
DC Power
Supply
Thomson’s Cathode Ray Tube
(1897)
+
Cathode
(-)
Anode
(+)
-
Second DC
Power
Supply
CRT Tube Video (Click to start)
Thomson’s experiment results
Beam was made of a particle 2000 times
smaller than the smallest atom (hydrogen)
Beam was negatively charged – attracted
to the positive electrode of a second
power supply. (electron)
Couldn’t find the corresponding positive
particle.
Assumed atom had a “fog” of positive
charge in which the electrons were
dispersed.
“Plum Pudding” Model
Some Information about Radiation
Alpha particles are large positively
charged particles; they are helium nuclei
Beta particles are smaller negatively
charged particles; they are electrons
Gamma rays are electrically neutral, high
energy electromagnetic radiation.
Rutherford’s Experiment
Radioactive α
Box
Animation
Rutherford’s Gold Foil Experiment
Trying to Prove Plum Pudding Model
Shot pos. (+) charged alpha particles at
thin gold foil.
Expected all to go straight through.
Most did go straight through but a few
(1 in 20,000) bounced off.
Gold Foil
Rutherford’s Astonishing
Conclusion
Atoms are mostly empty space !
There is a very dense, positively charged
nucleus that contains most of the mass.
Summary
Thomson and Rutherford
Thomson – the atom is made up of smaller
particles
Thomson – one of the smaller particles is
negatively charged
Rutherford - The atom has a dense
positive center.
Rutherford - The atom is mostly empty
space.
Rutherford Model
• Center – positive nucleus with neg.
electrons dispersed around nucleus
• Atom is mostly empty space.
Large Positive Nucleus
Empty space
Electrons
(randomly dispersed)
Along Comes Neils Bohr
Bohr Model
Electrons are in discreet energy levels
orbiting around the nucleus.
…And Then de Broglie - The Wave Model
 If electrons are particles and they orbit
the nucleus, why don’t they eventually
spiral into the nucleus?
 Electrons are waves not particles!
Electrons are everywhere around
The nucleus all at once.
 The Double Slit Experiment
Wave Model (de Broglie)
Electron are traveling at near the speed of
light.
Electrons are everywhere all at once in
their energy level or orbital around the
nucleus
Electrons are in standing waves of
potential around the nucleus at discreet
distances from the nucleus (Bohr)
Electrons as Waves –
The Double Slit Experiment. (Click to start)
The Atom and its Parts
Nucleus

Protons and Neutrons
Protons and neutrons are made up of quarks
Around the Nucleus

Electrons – # electrons equals # protons
when the atom has no charge.
In Between Electrons and Nucleus –
EMPTY SPACE !!!
Atomic number: Z, the number of protons in
the atom (Unique to an element, if the atomic
number is different the element is different)
Mass number: M, the number of protons and
neutrons in the atom.
Isotopes are atoms of the same element, ie
the have the same atomic number, but they
have a different mass number. They differ in
the number of neutrons. The chemical
properties of the isotopes are the same, but
they have different masses.
Notation
Mass Number
# of protons + neutrons
Atomic Number
# of protons
M
Z
E
Practice
Find the number of protons, neutrons and
electrons in the following elements
35
Cl
17
243
Am
95
Answers
35
17
Cl
243
95
Am
17 protons
17 electrons
35-17 = 18 neutrons
95 protons
95 electrons
243-95 = 148 neutrons
The Atom in Summary
Particle
Charge
Location in
Atom
9.1x10-28g
≈ 1/1000 amu
-1.6022x10-19C
(-1)
Outside the
nucleus
Proton
1.67 x10-24g
≈ 1 amu
+1.6022x10-19C
(+1)
Inside the
nucleus
Neutron
1.67 x10-24g
≈ 1 amu
0
Inside the
nucleus
Electron
Mass
(1amu=1.66x10-24g)
Atomic Mass



Definition: Mass of an atom of a particular
element in amu (atomic mass unit)
1 amu = 1.67 x 10-24 g (mass of a proton)
Average Atomic Mass: is the weighted
average of all of the atomic masses of the
isotopes in a naturally occurring sample of
the element ON THE PERIODIC TABLE
Some Useful Internet Sites
Cathode Ray Tube:
http://www.chem.uiuc.edu/clcwebsite/
video/Cath.avi
Gold Foil:
http://micro.magnet.fsu.edu/electromag/j
ava/rutherford/
Millikan Oil Drop,
http://www.physchem.co.za/Static%20El
ectricity/Millikan.htm
Law of Constant Composition (or
Definite Proportions):
Dalton - 1803
different samples of the same compound
always contain its constituent elements in
the same proportion by mass
Example:
Water always contains hydrogen and
oxygen in a mass ratio of 1:8 no matter
how it is formed or where it is found.
Law of Multiple proportions:
If two elements can combine to form more than
one compound, the masses of one element that
combine with a fixed mass of the other element
are in ratios of small whole numbers.
Example:
Water and Hydrogen peroxide both contain
hydrogen and oxygen:
Water H:O by mass is 1:8
Hydrogen peroxide is 1:16
Cathode Ray Tube Experiment
Thomson - 1897