2.1
Atoms and Their Structure
Objective
Relate historic experiments to the
development of the modern model of the
atom.
Illustrate the modern model of the atom.
Interpret the information available in an
element block of the periodic table.
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Early Ideas About Matter
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Our current model of the composition of
matter is based on hundreds of years of
work.
About 2500 years ago, the Greek
philosophers thought about the nature of
matter and its composition.
They proposed that matter was composed of
four fundamental elements…
And these were…
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Air
Water
Fire
Earth
Greek Philosophers
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They also debated whether matter
could be divided into smaller and
smaller pieces endlessly or whether
there was an ultimate smallest particle
that could no longer be divided.
They were excellent observers, but they
did not test their theories with
experiments.
Democritus, 460-370 B.C.
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Proposed that the world is made up a empty
space and tiny particles called atoms.
He thought that atoms were the smallest
particles of matter and that different types of
atoms exist for every type of matter.
Atomic Theory- matter is made up of
fundamental particles called atoms.
Antoine Lavoisier (1743-1794)
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French chemist
In 1782 he made
measurements of a
chemical change in
a sealed container.
Lavoisier’s Conclusion
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He observed that the mass of the reactants before
the chemical reaction were equal to the mass of
the products after the reaction.
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In a sealed container 2.0 g of hydrogen gas reacted with
16.0 g of oxygen gas to produce 18.0 g of water
Matter was neither created nor destroyed
during a chemical reaction, but changed.
This became known as the law of
conservation of matter
Law of Conservation of Mass
Joseph Proust
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Joseph Proust, French chemist
September 26, 1754 – July 5, 1826
Proust’s Contribution
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Observed that the composition of water is
always 11% hydrogen and 89% oxygen by
mass. Regardless of its source
He observed many other compounds and
observed that the elements that composed
the compounds were always in a certain
proportion by mass. This is referred to as
law of definite proportions.
John Dalton
Dalton’s Atomic Theory
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John Dalton (1766-1844), an English
schoolteacher and chemist
Studied the experiments of Lavoisier,
Proust, and many others.
Based on this he developed his atomic
theory.
Dalton’s Atomic Theory
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All matter is made up of atoms.
Atoms are indestructible and cannot
be divided into smaller particles(Atoms
are indivisible)
All atoms of one element are exactly
alike, but they are different from
atoms of other elements.
Dalton’s Atomic Theory
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This gave chemist a model of the
particle nature of matter, but it also
raised a lot of questions.
If all elements are made up of atoms
then why are there so many different
types of elements?
What makes one atom different from
another?
Dalton’s Atomic Theory
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Experiments performed late in the 19th
century began to explain the properties
and behavior of substances
This was done by the discovery of three
smaller particles.
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Protons, electrons and neutrons
Atomic Theory, Conservation
of Matter and Recycling
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What happens to the stuff you throw
away? What happens to the atoms?
As you have already learned matter is
neither created nor destroyed, so what
happens.
What happens?
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When waste is incinerated or buried the
atoms may combine with oxygen and
other substances to form new
compounds.
The atoms are “recycled”
Recycling
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Recycling has become a part of life
now.
Much of what you buy is either recycled
or can be recycled.
We have found the advantage of
recycling materials and therefore
atoms. We mimic what nature does
and conserve our natural resources.
Hypotheses
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Hypothesis – testable prediction to
explain observations.
Hypotheses are based on observations.
They can be proven correct or incorrect
by the experiments that are designed to
test them.
Hypothesis
Theories
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Theory- an explanation based on many
observations and supported by the
results of many experiments.
As scientists gather new information, a
theory may be revised or replaced.
Theory
Laws
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Scientific Law – a fact of nature that is
observed so often that it is accepted as the
truth.
Examples:
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Sun rises in the east
A law can generally be used to make
predictions but does not explain why
something happens.
Theories explain laws.
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One part of Dalton’s atomic theory explains why
the law of conservation of matter is true.
Review
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What is the difference between a theory
and a hypothesis?
What is the Law of Conservation of
Mass?
The Discovery of Atomic
Structure
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Dalton’s atomic theory was almost true.
He assumed that atoms are the ultimate
particles of matter and can’t be broken up
into smaller particles and that all atoms of the
same element are identical.
His theory needed to be modified with the
discovery of electrons, protons and neutrons.
The Electrons
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In 1897, JJ Thomson a British physicist
discovered that the solid ball of model
was not accurate.
His experiments involved the use of a
cathode ray.
Cathode Ray
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Composed of a vacuum tube
At the end of the tube is a piece of metal
called on electrode which is connected to a
metal terminal on the outside of the tube.
The electrode becomes electrically charged
when connected to a high voltage.
When the electrode is charged rays travel in
the tube from the negative electrode(the
cathode) to the positive electrode (the
anode)
Cathode Ray
Cathode Ray
Thomson’s Discovery
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What he found that the ray would bend
towards a positive charged plate and
away from a negatively charged plate.
He concluded that the rays are
composed of invisible negatively
charged particles he called electrons
Electron – negatively-charged particle
Early Atomic Model
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The early atomic model was referred to
as the “plum pudding model”; you could
more closely relate it to a chocolate
chip cookie
Scientists believed that atoms were
balls of positive charge with the
negatively charged particles embedded
in them.
Changes in the Atomic Model
Solid Ball
Model
Plum Pudding
Model
Ernest Rutherford
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In 1909 he carried
out the first of the
experiments that
would reveal an
arrangement far
different from the
“plum pudding”
model
Rutherford’s Experiment
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Gold Foil Experiment
He set up a lead-shielded box containing
polonium, which emitted a positively charged
beam of alpha particles.
When the beam struck a sheet of gold foil,
most of the particles passed straight through
the foil
However, some of the particles from the
beam were deflected. Some were only
slightly deflected and some bounced straight
back.
Gold Foil Experiment
Gold Foil Experiment
The Nuclear Model of the
Atom
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Based on Rutherford’s work the team
devised a new model for the atom.
Because some of the particles bounced
straight back they concluded that atoms
must have a dense central core called
the nucleus
Nucleus – a small, dense, positively
charged central core of an atom
Nuclear Model
Changes in the Atomic Model
Solid Ball
Model
Plum Pudding
Model
Nuclear
Model
Atomic Model
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It was hard for people to grasp that atoms
contained a lot of empty space.
When they looked at a rock, it was very
difficult to see how most of this object could
be empty space.
If you were to enlarge an atom of hydrogen
so the nucleus was the size of a golf ball, the
electron would be a mile a way.
In one drop of water there is
6,500,000,000,000,000,000,000 (6.5
sextillion) atoms
Atomic Numbers and Masses
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The nucleus of an atom is composed of
protons and neutrons. Electrons move
in the space around the nucleus.
Atomic Number – the number of
protons in the nucleus of an atom.
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Every element has a unique atomic
number. Therefore, it is the number of
protons that determines the identity of an
element.
Atoms
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Protons - positively charged subatomic
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Neutrons – subatomic particle that does not
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particles.
have a charge, it is neutral
Atoms have no overall charge.
Therefore the number of protons and
electrons must be equal.
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If you elements atomic number is 2
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2 protons
2 electrons
Mass Number
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Mass number - the number of protons
and neutrons in the nucleus of an atom.
Isotope – atom of an element with a
different number of neutrons and
therefore a different mass number
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Ne-20 = neon 20(10 protons, 10 neutrons)
Ne-21 = neon 21(10 protons, 11 neutrons)
Neon
Composition of Atoms
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You can determine
the composition of
any element if you
know the
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Atomic number
Mass number
Atomic Mass
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Since elements have different isotopes the
atomic mass is the weighted average.
Located under the symbol on the periodic
table.
You can use the atomic mass to determine
the mass number of a element
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You take the atomic mass rounded up
Determining Atomic Mass
62.930 X 0.6917 = 43.529 43.529 + 20.017 = 63.546
64.928 X 0.3083 = 20.017
Review
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What information does the atomic
number give us?
How do we determine the number of
neutrons of any element?
Who discovered the electron?
What discovery did the Gold Foil
Experiment led to?