Chapter 2.1-2.2 The Discovery of the Atomic Structure
Transcript of Chapter 2.1-2.2 The Discovery of the Atomic Structure
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The Discovery of the Atomic Structure
Advanced Chemistry
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Early Models of the Atom
• An atom is the smallest particle of an element that retains its identity in a chemical reaction.
• Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.
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Democritus (460-370 BC)
•Described the material world as made up of tiny indivisible particles they called atomos.
• Indivisible or uncuttable
•Plato & Aristotle believed there can be no ultimately indivisible particles.
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John Dalton
• Idea of atoms reemerged in Europe during 17th century.
• Chemists could measure the amount of elements that reacted with one another to form new substances.
• John Dalton developed the atomic theory that was based on four postulates
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Dalton's Postulates
Each element is composed of extremely small particles called atoms.
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Dalton's Postulates
All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.
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Dalton's Postulates
Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
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Dalton's Postulates
Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.
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Laws of Chemical Combination
• Dalton explains several laws of chemical combination in his atomic theory.
1. Law of constant composition
2. Law of conservation of mass
3. Law of multiple proportions
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Law of Constant Composition
In a given compound, the relative numbers and kinds of atoms are
constant.
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Law of Conservation of Mass
The total mass of materials present after a chemical reaction is the same as the total mass
present before the reaction.
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Law of Multiple Proportions
If two elements A and B combine to form more than one compound, the masses of B that can combine
with a given mass of A are in the ratio of small whole numbers.
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Much of Dalton’s atomic theory is accepted today.
• One important change, however, is
that atoms are now known to be
divisible.
• They can be broken down into even
smaller, more fundamental particles,
called subatomic particles.
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Three kinds of subatomic particles are electrons, protons, and neutrons.
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In 1897, the English physicist J. J. Thomson (1856–1940) discovered the electron.
• Electrons are negatively charged
subatomic particles.
Electrons
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ElectronsThomson performed experiments that involved passing electric current through gases at low pressure.
• He sealed the gases in glass tubes fitted at
both ends with metal disks called
electrodes.
• The electrodes were connected to a source
of electricity.
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Electrons
• One
electrode,
the anode
became
positively
charged.
• The other electrode, the cathode, became
negatively charged.
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Electrons
The result was
a glowing
beam, or
cathode ray,
that traveled
from the
cathode to the
anode.
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Electrons
Thomson found
that a cathode
ray is deflected
by electrically
charged metal
plates.
• A positively charged plate attracts the cathode
ray, while a negatively charged plate repels it.
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Thompson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed.
• Thompson called these particles
corpuscles.
• Later they were named electrons.
Electrons
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The U.S. physicist Robert A. Millikan (1868–1953) carried out experiments to find the quantity of an electron’s charge.
• In his oil-drop experiment, Millikan
suspended negatively charged oil droplets
between two charged plates.
• He then changed the voltage on the plates
to see how this affected the droplets’ rate of
fall.
Electrons
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The U.S. physicist Robert A. Millikan (1868–1953) carried out experiments to find the quantity of an electron’s charge.
• From his data, he found that the
charge on each oil droplet was a
multiple of 1.60 10–19 coulomb,
meaning this must be the charge of
an electron.
Electrons
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Radioactivity
•Radioactivity is the spontaneous emission of radiation by an atom.
• It was first observed by Henri Becquerel.
•Marie and Pierre Curie also studied it.
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Radioactivity• Three types of radiation were discovered by
Ernest Rutherford:
• particles – attracted to negatively charged plate
• particles – attracted to positive charged plate
• rays – carries no charge
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When subatomic particles were discovered, scientists wondered how the particles were put together in an atom.
• Most scientists—including J. J. Thompson—
thought it likely that the electrons were evenly
distributed throughout an atom filled uniformly
with positively charged material.
– In Thomson’s atomic model, known as the “plum-
pudding model,” electrons were stuck into a lump
of positive charge, similar to raisins stuck in
dough.
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Plum Pudding Model
This model of the atom turned out to be short-lived, however, due to the work of a former student of Thomson, Ernest Rutherford (1871–1937).
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Rutherford’s Gold-Foil ExperimentIn 1911, Rutherford and his co-workers wanted to test the existing plum-pudding model of atomic structure.
• They devised the gold-foil experiment.
• Their test used alpha particles, which are helium
atoms that have lost their two electrons and
have a double positive charge because of the
two remaining protons.
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Rutherford’s Gold-Foil Experiment
In the experiment, a narrow beam of alpha particles was directed at a very thin sheet of gold.
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Rutherford’s Gold-Foil Experiment
In the experiment, a narrow beam of alpha particles was directed at a very thin sheet of gold.
• According to the prevailing
theory, the alpha particles
should have passed easily
through the gold, with only
a slight deflection due to
the positive charge thought
to be spread out in the gold
atoms.
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Copyright © Pearson
Education, Inc., or its affiliates. All
Rights Reserved.
Rutherford’s Gold-Foil Experiment
Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.
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Copyright © Pearson
Education, Inc., or its affiliates. All
Rights Reserved.
Rutherford’s Gold-Foil Experiment
Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.
• What was surprising
is that a small
fraction of the alpha
particles bounced off
the gold foil at very
large angles.
• Some even bounced
straight back toward
the source.
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Based on his experimental results, Rutherford suggested a new theory of the atom.
• He proposed that the atom is mostly empty
space.
– Thus explaining the lack of deflection of most
of the alpha particles.
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Based on his experimental results, Rutherford suggested a new theory of the atom.
• He concluded that all the positive charge and
almost all of the mass are concentrated in a
small region that has enough positive charge to
account for the great deflection of some of the
alpha particles.
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Neutrons
In 1932, the English physicist James Chadwick (1891–1974) confirmed the existence of yet another subatomic particle: the neutron.
• Neutrons are subatomic particles with no
charge but with a mass nearly equal to that
of a proton.