Download Mileposts on the road to the atom

Atoms: discovering the
stuff that stuff is made of
We know all matter is made from elements
What makes each element unique?
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A component unique to each element?
A common component but assembled in a different
way?
Learning Objectives
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Describe what is meant by “atom”
Describe Law of Conservation of Mass and
Definite Proportions
Identify main features of Dalton’s atomic
theory
Describe Avogadro’s Hypothesis
Glimpse into the interior

What we “see” now
with scanning tunneling
microscope could only
be speculated on 2,000
years ago
400 BC
Proposal that matter is particulate
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Democritus posed the question: could matter be
subdivided forever? He answered no: there is a limit
to the extent to which matter can be subdivided, and
he coined the term atom from the Greek for
uncuttable a-tomos.
Democritus’ legacy
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No indication about the size of these atoms. In
fact Democritus atoms could be extremely
large
Democritus’ idea has endured with some
modifications:
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Atoms are not uncuttable
Atoms are all very small
Atoms are themselves made of particles
Science in the dark ages
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The decline of Greek civilization saw a concomitant
decline of intellectual activity in Europe
The major “scientific” activity was alchemy, largely
the pursuit of the transformation of matter into gold
Uncritical acceptance of Greek thinking about matter
lingered until the Age of Enlightenment
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Matter is made of the four elements (earth, wind and
fire...and water)
The modern scientific era began in the 17th century,
pioneered by a few inspired individuals who broke
free from long-established conventions. Modern
science is based on observation and reason
1780
Re-emergence of the atom:
Daniel Bernoulli’s kinetic theory of gases
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Atoms were assumed to be hard round
spheres which behaved much like billiard
balls.
Success of kinetic theory in describing gases
lends support to atomic description of matter.
It is still used, with scarcely any
modification
1790
Order out of chaos
Antoine Lavoisier defines elements: substances that
could not be broken down into simpler substances.
 Beginnings of periodic table
and definition of elements
 Introduced concept of
compounds (combination of
elements)
 Demonstrated Law of
Conservation of Mass
 Discredited phlogiston theory
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No additional insights into the
atom at this stage.
Chemical composition laws and
arguments for an atomic world
Law of Conservation of Mass:
Matter is neither created nor destroyed in the course of a
chemical reaction.
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Does not apply to nuclear changes
Law of Definite Proportions:
In forming compounds, elements combine together in
definite mass ratios
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No knowledge of actual atom ratios at this stage, but how
else to explain fixed quantities unless the elements were
present as discrete bodies
Strong indication for combination of the atoms in simple
ratios
1803
John Dalton’s atomic musings
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Sample of any element contains tiny
particles called atoms
Atoms cannot be subdivided, created or
destroyed
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Law of Conservation of Matter
All atoms of the same element are the
same
All atoms of different elements are
different
Atoms combine together in simple
whole number ratios
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Law of Multiple Proportions: The ratio of the masses
of one element combined with the same mass of
another element is a simple whole number
Significance of the Law of Multiple
Proportions
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With benefit of knowledge of chemical
composition, consider example of carbon
dioxide (CO2) and carbon monoxide (CO)
MassOCO2
MassOCO
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2
Combination of finite components (atoms) of
C and O in simple number ratios is the most
sensible explanation of the Law
1808
Gay-Lussac law of combining
volumes: atomic implications
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Gases react with other gases
to give products, in volumes
which have simple whole
number ratios.
“Mystery” of the reaction of
hydrogen and oxygen to
provide water.
2 vols of hydrogen + 1 vol
of oxygen gives 2 vols of
water (Why not 1 vol?)
The ratio of 2:1 was
inconsistent with Dalton’s
belief that formula of water
was HO (principle of
simplicity)
Mystery of the gas volumes
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Combining H and O in 1:1 ratio (Dalton’s
proposed ratio) does not satisfy Conservation
of Matter…
Needs another box of O atoms
H:O = 2:1
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However, the 2:1 ratio (as suggested by
volume ratio of reactants) predicts only 1 vol
of H2O, not 2 vols as observed…?
Enter Avogadro:
What if hydrogen and oxygen are
diatomic molecules?
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Matter conserved, and each volume contains
same number of particles
Seems perfect right? Dalton couldn’t accept
either formula of water or atoms of same
element combining
1811
Amadeo Avogadro’s hypothesis
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Hypothesis that same volumes of all gases
contain same number of particles.
Results conflicted with Dalton’s views and were
not recognized for nearly 50 years
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Solved riddle of gas volumes by positing
that gas molecules contained two atoms.
Most elemental gases are diatomic
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Key result: Able to calculate the relative
atomic masses of the elements:
development of a scale of atomic weight.
1815
William Prout weights of atoms appear to be
simple multiples of the hydrogen atom.
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Coincidence or significance?
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This led to the proposal that atoms of heavier
elements were made from hydrogen atoms
Implication that larger atoms comprise smaller units
Partial truth: there are common factors between atoms
of different elements…
but they are not H atoms…
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Development of atomic weight scale
– elimination of the missing link
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Experimentally mass could be measured
Without knowledge of atomic mass, impossible to
know how many atoms of one element combine with
another
Essential to know number of atoms to understand
chemistry – write chemical formulae
Atomic weight scale, largely enabled by Avogadro,
provides link between experimental observables and
numbers of atoms
Unanswered questions
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What features of the atoms are responsible for
the differences in element properties?
How can atoms actually bind together in
compounds to give substances of different
properties if atoms cannot be modified? What
are the attractive forces?
What about those other experimental
observations accumulating…?