Atoms: discovering the
stuff that stuff is made of
We know all matter is made from elements
What makes each element unique?A component unique to each element?
A common component but assembled in a different way?
Describe what is meant by atom
Describe Law of Conservation of Mass and
Identify main features of Daltons atomic
Describe Avogadros Hypothesis
Glimpse into the interior
What we see now
with scanning tunneling
microscope could only
be speculated on 2,000
Proposal that matter is particulate
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
Democritus idea has endured with some
Atoms are not uncuttable
Atoms are all very small
Atoms are themselves made of particles
Science in the dark ages
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 Matter is made of the four elements (earth, wind and
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
1780Re-emergence of the atom:
Daniel Bernoullis kinetic theory of gases
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
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
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. Does not apply to nuclear changes
Law of Definite Proportions:
In forming compounds, elements combine together in definite mass ratios 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
John Daltons atomic musings Sample of any element contains tiny
particles called atoms
Atoms cannot be subdivided, created or destroyed 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 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
With benefit of knowledge of chemical composition, consider example of carbon dioxide (CO2) and carbon monoxide (CO)
Combination of finite components (atoms) of C and O in simple number ratios is the most sensible explanation of the Law
Gay-Lussac law of combining
volumes: atomic implications 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 Daltons belief that formula of water was HO (principle of simplicity)
Mystery of the gas volumes
Combining H and O in 1:1 ratio (Daltons
proposed ratio) does not satisfy Conservation
Needs another box of O atoms
H:O = 2:1
However, the 2:1 ratio (as suggested by
volume ratio of reactants) predicts only 1 vol
of H2O, not 2 vols as observed?
What if hydrogen and oxygen are
diatomic molecules? Matter conserved, and each volume contains
same number of particles
Seems perfect right? Dalton couldnt accept
either formula of water or atoms of same
Amadeo Avogadros hypothesis
Hypothesis that same volumes of all gases
contain same number of particles.
Results conflicted with Daltons views and were
not recognized for nearly 50 years
Solved riddle of gas volumes by positing
that gas molecules contained two atoms.
Most elemental gases are diatomic
Key result: Able to calculate the relative
atomic masses of the elements:
development of a scale of atomic weight.
William Prout weights of atoms appear to be
simple multiples of the hydrogen atom.
Coincidence or significance?
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
Development of atomic weight scale
elimination of the missing link
Experimentally mass could be measured
Without knowledge of atomic mass, impossible to
know how many atoms of one element combine with
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
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