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Definition & Purpose
of Scientific Models
What is meant by the scientific term “Model”?
A model is a visual representation of
an idea, concept or structure.
What is the purpose of a “Scientific Model”?
A model is used to help make the
concept, idea or structure
more understandable.
Developing Scientific Models
Models are developed through the application
of two types of Observations
Direct:
The observer is present during the event, witnessing
it first hand.
Indirect:
The observer is not present during the event and
must make inferences from the data present.
An inference is a conclusion made after
considering all indirect evidence gathered.
Using Scientific Models
Give 2 examples of Models used in the
following settings:
The Home:
Business & Technology:
Medicine:
The Science Classroom:
Using Scientific Models
Name 2 Careers that would use data/evidence
gathered by using direct observations:
Name 2 Careers that would use data/evidence
gathered by using indirect observations:
Scientific
Models
How did scientists use indirect and direct
observation skills to gather the data
necessary to develop what we now accept
as our current model of the atom?
Hopefully, the following activity
will help your understanding.
Scientific
Models
Observe the tests performed
on the Mystery Tube and then
describe & draw them using
the handout provided.
Hypothesis #1
Scientist credited with discovery: ____________________
Hypothesis #2
Scientist credited with discovery: ____________________
Hypothesis #3
Scientist credited with discovery: ____________________
Connections to the knowledge
we now have about the atom:
The “Mystery Tube” is like studying the atom
because…
1. ________________________________
2. ________________________________
3. ________________________________
Early Atomic Models
• Atoms are so small they cannot be observed directly.
Scientists can only use the data gathered experimentally
making inferences using indirect observations.
•
Around 400 B.C., Democritus (a Greek philosopher) suggested
that the world was made of two things, empty space and tiny
particles called atoms.
•
During the 1800's, a French Chemist (Antoine Lavoisier) discovered
that chemical "changes” occurring in a closed system - the mass
after a chemical change was equal to the mass before the chemical
change.
One of the first Atomic Models
The First
formal Atomic Model
Air
Fire
(Aristotle & the
Early Greeks, 390BC)
Water
Earth
FOUR ELEMENT THEORY
• Plato was an atomist (380BC)
• Thought all matter was
composed of 4 elements:
–
–
–
–
–
Earth (cool, heavy)
Water (wet)
Fire (hot)
Air (light)
Ether (close to heaven)
FIRE
Hot
Dry
‘MATTER’
AIR
Wet
EARTH
Cold
WATER
Relation of the four elements and the four qualities
Blend these “elements” in different proportions to get all substances
The Evolution of a Model
Democritus (400 B.C.)
• Proposed that matter was
composed of tiny indivisible
particles
• Not based on experimental
data
• Greek: atomos
Alchemy
(approx. next 2000 years, (100BC -1800)
• Mixture of science and mysticism.
• Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.
John Dalton (1807)
Considered the “Father of the Atomic Theory.”
• British School teacher
– based his theory on others’
experimental data
• Billiard Ball Model
– atom is a
uniform,
solid sphere
John Dalton, cont.
Four Postulates of Dalton’s Theory:
1. Elements are composed of small indivisible
particles called atoms.
2. Atoms of the same element are identical.
Atoms of different elements are different.
3. Atoms of different elements combine together
in simple proportions to create a compound.
4. In a chemical reaction, atoms are rearranged,
but not changed.
John Dalton, cont.
• Dalton stated a second law based on his own atomic theory
but not based on experimental data. It concerns elements
that form more than one compound with each other.
For example: Oxygen can combine in a 1:1 ratio by
mass with Carbon to form Carbon
Monoxide, CO, or a 1:2 ratio by mass
to form Carbon Dioxide, CO2.
According to Dalton's second law, the ratio of masses of
one element that combine with a constant mass of another
element can be expressed in whole numbers (or %) - This
statement is known as the Law of Multiple Proportions.
Henri Becquerel & Marie Curie, (1896)
• Discovered radioactivity
– spontaneous emission of
radiation from the nucleus
• Three types:
– alpha () - positive
– beta () - negative
– gamma () - neutral
Particle I.D. using penetrating ability
Alpha Waves: Made of positive particles which can be
stopped by paper.
Beta Waves: Made of negative particles which can be
stopped by heavy clothing.
Gamma Waves: Made of very high non-particle radiation
which can be stopped only by lead.
TYPES OF RADIATION:
If the structure of the nucleus is not stable, the nucleus will begin
to decompose by ejecting a particle of energy until it reaches a
more stable arrangement.
Alpha Rays
Consist of 2 protons and 2 neutrons with no electrons (positively
charged), the are made up of Alpha particles which have a speed
of one-tenth the speed of light.
Beta Rays
Electrons with higher kinetic energy than electrons emitted by
Cathode Ray Tubes. They are composed of neutrons which
have changed to a proton & an electron.
Gamma Rays
More energy than x-rays; high frequency & short wavelength.
Albert Einstein, (Early 1900’s)
Explained the origin of the energy released during
nuclear changes.
Einstein hypothesized that there is energy contained
within the matter contained in the nucleus of every
atom. This statement can be expressed in the famous
equation:
E = mc2
E = energy (in joules)
m = mass (in kilograms)
c = the speed of light (meters/second)
J. J. Thomson (1903)
• Cathode Ray Tube
Experiments
– beam of negative particles
• Discovered Electrons
– negative particles within
the atom
• Plum-pudding Model
J. J. Thomson (1903)
Experiments by several scientists in
the middle 19th century led to the
conclusion that the atom was made
up of several smaller particles.
With the use of a device called a
Cathode-Ray tube it is possible to see
these particles.
J. J. Thomson (1903)
In each end of the tube there is an
electrode. When connected to a
source of high voltage electricity,
the electrodes become charged.
The positive electrode is called the anode.
The negative electrode is called the cathode.
The Cathode Ray Tube
The Cathode Ray Tube
Careful observations revealed rays produced inside the
tube. Because the rays appeared to begin at the cathode and
travel toward the anode, the rays were called cathode-rays.
Beam
Cathode ray tubes are used as “picture tubes”
in televisions and computer monitors.
The Cathode Ray Tube
Cathode ray tubes with a fluorescent screen at one end
would glow. Thomson measured the deflection of the beam. A
Magnet deflected the beam in one direction and a plate
attracted the beam after it was deflected.
Beam
The Cathode Ray Tube
Beam
Looking at the diagram above, what do you think
Thompson concluded about the composition of the beam?
The beam was made of negative particles (electrons).
The Cathode Ray Tube
Thompson’s experiment showed that the deflection
of a charged particle was dependent upon:
 The mass of the particle.
 The speed of the particle.
 The electrical charge of the particle.
 The strength of the magnetic field
 The amount of charge on the plates
The Cathode Ray Tube
Regardless of the type of gas or metal used in cathoderay tube, the ratio of charge to mass remained the
same. Therefore, what do you think he concluded?
The particles in the cathode-ray tube
were identical to one another.
The symbol for the electron is e- , indicating
that the electron has a negative charge.
The Cathode Ray Tube
Thomson also observed the following when he used
hydrogen gas and high voltage with low pressure in
cathode-ray tube - he noticed that two beams (one
negative) and another beam moving in the opposite
direction toward the cathode - a positive beam.
Thomson found that the deflection of the beam
varied with different gases. Hydrogen ions had the
greatest deflection, therefore the smallest mass.
A Proton is a positively charged particle
found in all atoms possessing a +1 charge.
The Cathode Ray Tube
J. J. Thomson (1903)
Plum-pudding Model
– positive sphere (the
pudding) with
negative electrons (the
plums) dispersed
throughout.
Ernest Rutherford (1911)
• Gold Foil Experiment
• Discovered the nucleus
– dense, positive charge in
the center of the atom
• Nuclear Model
Rutherford’s Gold Foil Experiment
Alpha particles (in a beam) are shot at gold, platinum, copper, and
tin foil with a fluorescent screen around the sheet of foil and found
that:
Most particles went straight through
the foil.
Some particles paths were altered but still
went through the foil.
Some particles were deflected and
headed right back at the source.
What can be concluded by these results?
Isotopes and Atomic Number
While working with Neon, Thomson observed what
seemed to be two kinds of neon atoms. They were
exactly alike chemically, but different in their masses.
Atoms of the same element that differ in mass are
called Isotopes. Isotopes have the same number of
protons but a different number of neutrons which
accounts for the difference in their masses.
Isotopes and Atomic Number
The number of protons determines the identity of
the element and the number of neutrons
determines the isotope of the element.
The number of protons found in the nucleus is
equal to the atomic number of the element.
Isotopes and Atomic Number
The total number of protons and neutrons is called
the mass number ( = to atomic mass) of that atom.
To find the number of neutrons in an atom: (Atomic
mass - Atomic number) = # Neutrons
Unstable Isotopes are radioactive.
Ernest Rutherford (1911)
• Nuclear Model
– dense, positive nucleus surrounded
by negative electrons
Niels Bohr (1913)
• Bright-Line Spectrum
– tried to explain presence
of specific colors in
hydrogen’s spectrum
• Energy Levels
– electrons can only exist in
specific energy states
• Planetary Model
Niels Bohr (1913)
Bright-line spectrum
• Planetary Model
– electrons move in circular
orbits within specific
energy levels
Erwin Schrödinger (1926)
• Quantum mechanics
– electrons can only exist in
specified energy states
• Electron cloud model
– orbital: region around the
nucleus where e- are likely
to be found
Erwin Schrödinger (1926)
Electron Cloud Model (orbital model)
• dots represent probability of
e- location, not actual electrons
James Chadwick (1932)
• Discovered neutrons
– neutral particles in the
nucleus of an atom
• Joliot-Curie
Experiments
– based his theory on their
experimental evidence
James Chadwick (1932)
Neutron Model
• Revision of Rutherford’s Nuclear Model
Current Quantum Model
Quantum Model Animation
Current Quantum Model
2s Orbital
Nucleus
2p
Orbital
1s Orbital
3s Orbital
Quantum Model
• Result of the collective work of many scientists
Current Quantum Model
Gellmann’s Quarks (Murray Gellmann, 1969)
• Smaller particles making up protons
Quantum Mechanical Theory
Summary of our current understanding of the Atom
•
Atom is mostly empty space.
•
Dense nucleus containing protons & neutrons;
nucleus has a net + charge.
•
Electrons found in the “cloud” surrounding the
nucleus; location determined by probability based
on electron density.
Electrons are located in quantum (energy)
levels and in orbitals within each quantum level.
•
•
Shape & size of the atom is determined by the
orbitals found within the atom.
•
This is shown (supported) by the periodic table.