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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
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
Identify main features of the periodic table
Glimpse into the interior
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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
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
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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 - 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: The ratio of
the masses of one element combined with
the same mass of another element is a
simple whole number
Significance of Law of Multiple
Proportions for atomic world
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Illustration using carbon dioxide (CO2) and
carbon monoxide (CO)
MassOCO2
MassOCO
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2
Combination of C and O as atoms in simple
number ratios is most sensible explanation
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…
H:O = 2:1
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However, 2:1 ratio (Gay Lussac’s expt)
predicts only 1 vol of H2O, not 2 vols as
observed. Hmmm...
Enter Avogadro:
What if gases 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 are simple
multiples of hydrogen atom.
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Coincidence or significance?
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Proposed 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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Atomic weight scale – the link
between mass and number of atoms
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Experimentally mass could be measured
Without knowledge of atomic mass, impossible to
know how many atoms of one element combines with
another
Essential to know number of atoms to understand
chemistry
Atomic weight scale, enabled by Avogadro’s
hypothesis, provides link between experimental
observable (mass) and numbers of atoms
1869 - elements ordered
Dimitri Mendeleev and the periodic table
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The most important document in
chemistry
Ordered the elements according
to their atomic weights and
properties.
Only 60 elements identified at
the time
Predicted existence of
undiscovered elements and their
properties which were often
proved startlingly accurate.
Facts of periodic table
There are 91 naturally occurring elements: 2 liquids, 11 gases,
23-25 nonmetals
The Periodic Table: Groups and Periods
Groups: columns of
elements
Periods: rows of
elements
Group (family) behaviour
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1A ALKALI METALS: Reactive. Reactivity
increases down group. Physical properties vary in a
gradual fashion.
2A ALKALINE EARTH METALS : Similar to
alkalis but less reactive. Lots of common calcium
compounds: calcium carbonate (limestone, marble),
calcium oxide (lime), calcium sulphate (gypsum,
plaster of Paris)
Important groups on nonmetal side
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7A HALOGENS: Very reactive lots of compounds
with most elements. Chlorides in particular are
common: NaCl. KCl. Reactivity decreases down
group
8A RARE, INERT, NOBLE GASES: All gases,
discovered late because of lack of compounds. Not
rare - 1 % argon in air; helium is second most
abundant element in universe. Very unreactive.
Unanswered questions
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What features of atoms are responsible for
differences in element properties?
How can atoms actually bind together in
compounds; what are the attractive forces?
What about those other experimental
observations accumulating…?