Download Uncovering the atom

Mileposts on the road to
the atom
All matter is made from elements
What makes elements unique?
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A component unique to each element
A common component but assembled in a different way
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.
No indication about the size of these atoms. In fact
Democritus atoms could be extremely large
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 lingered until
the Age of Enlightenment
The modern scientific era began in the 17th century,
pioneered by a few inspired individuals
1780
Daniel Bernoulli’s development of
the kinetic theory of gases based on
the concept of atoms
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Atoms were assumed to be hard round
spheres which behaved much like billiard
balls. The kinetic theory of gases, which
was developed using this hypothesis, was
remarkably successful in describing the
experimental behaviour of gases. It is still
used, with scarcely any modification, today
1790
Antoine Lavoisier compiled a list of elements:
substances that could not be broken down into
simpler substances.
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The beginning of assembly of the periodic
table and the notion that there are some
materials (elements), from which all the other
substances are derived.
Introduced concept of compounds
Demonstrated Law of Conservation of Mass
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No additional insights into the atom at this stage.
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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
- musing on atoms:
John Dalton’s atomic theory
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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
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 for gases.
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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
unproven belief that the formula of water was HO
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…
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 they are diatomic molecules?
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Matter conserved, and each volume contains
same number of particles
1811
Amadeo Avogadro’s hypothesis
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Solved riddle of gas volumes by positing that the
molecules in the gas contained two atoms.
Most elemental gases are diatomic
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Second part of hypothesis was that the same volumes
of all gases contain the same number of particles.
Results conflicted with Dalton’s views and were not
recognized for nearly 50 years
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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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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 the atoms cannot be modified
what would the attractive forces be?
What about those other experimental
observations accumulating…?