A BRIEF HISTORY OF MODELS OF THE ATOM

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A BRIEF HISTORY OF MODELS OF THE ATOM
2b Models in Science and Religion
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A Brief history of (models of) the atom1 The
Greeks

• The first people to suggest that the world was
  composed of atoms (Greek atomos, literally
  indivisible) were the Greek thinkers Leucippus
  and his disciple Democritus in the 5th cc BC.
  Democritus argued that atoms were solid, hard,
  incompressible and indestructible. Atoms existed
  not only for matter but for perception. Sourness
  for example, was caused by needle shaped atoms.

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A Brief history of (models of) the atom2 The
emergence of experimental science

• In the 17th and 18th centuries a number of
  individuals, who were important in the beginnings
  of what we would now call experimental science,
  advocated some form of atomic theory. Gassendi
  argued that God created atoms. Boyle said that
  atoms differ in shape, size, motion or rest, and
  texture. Newton wrote in the Opticks, God in
  the beginning formd Matter in solid, massy,
  hard, impenetrable, moveable Particlesno
  ordinary Power being able to divide what God
  himself made.

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A Brief history of (models of) the atom3 The
beginning of modern atomic theory

• Dalton, in the first decade of the 19th cc,
  extended the work of Proust and produced the
  first application of atomic theory to chemistry.
  He argued that molecules of a single element are
  atoms of the same fixed mass. Also, his law of
  multiple proportions showed how, using atomic
  theory, compounds are formed with the integer
  ratios of constituent elements. Atoms were joined
  together by forces. This work was extended by Gay
  Lussac, Avogadro, Cannizzaro and others. Note
  that not every idea of these men was found to be
  subsequently correct.

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A Brief history of (models of) the atom4 The
Kinetic theory of gases

• Interestingly, although Kinetic theory had been
  independently developed by Bernoulli, Herepath
  and Waterston it was many years before it was
  widely accepted. The caloric theory held sway for
  many years. Caloric was a hypothetical weightless
  fluid in substances that could turn into heat.
  Only when Joule showed that heat was a form of
  energy in the 1850s did caloric theory collapse.
  Clausius, Boltzmann and Maxwell were able to
  fully develop kinetic theory based on particles
  in motion.

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A Brief history of (models of) the atom5 The
Periodic Table

The power of atomic theory became evident when
Mendeleyev published his periodic table of the
elements in 1869. He built on the insights of
Avogadro and Cannizzaro and arranged the 63 then
known elements into a pattern. He was able to
predict the existence of new and unknown elements
where gaps appeared in the table. Here was a
fruitful example of the power of models. What his
work did was to raise an important question for
later for atomic theory, namely where did atomic
weights come from?
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A Brief history of (models of) the atom6 At
the start of the 20th century

Atoms were known to be electrically neutral, so
the discovery in 1897 of the electron by Thomson
posed questions about the structure of the atom.
One proposal was the plum pudding model devised
by Kelvin in 1902 which Thomson liked. The
positive charge was uniformly distributed and the
electrons embedded in it. An alternative was the
Saturnian model of Nagaoka with electrons
orbiting a positive center like rings around
Saturn. But Hertz had shown in 1887 that
accelerating charges radiate energy, so this
model was unstable.
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A Brief history of (models of) the atom7
Rutherfords model

Rutherfords interpretation of the alpha-particle
scattering experiment showed that a new model of
the atom was needed. The positive charge had to
be concentrated in a tiny nucleus and the
electrons had to be in some region around this
nucleus. He famously said about the unexpected
scattering that it was as unexpected as firing a
fifteen inch naval shell at tissue paper and it
bouncing back to hit you. But orbiting electrons
could not be stable - they would radiate energy.
This model accounted for the nucleus but not the
electron structure.
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A Brief history of (models of) the atom8 X-ray
vision

Moseley and Bohr working with X-rays and
analysing spectra led to Bohr in 1913 proposing a
quantised shell model of electrons in atoms. This
gave them stable orbits, but as yet there was no
theory of why this was so and Bohrs theory only
worked for hydrogen and failed to explain some
know properties of hydrogen spectra. Despite
modifications to the Bohr model by others, it was
clearly inadequate and by the 1920s needed
replacing.
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A Brief history of (models of) the
atom9 Enter Quantum Mechanics

Dramatic conceptual developments enabled the
creation of a model that was consistent with all
of the experimental data for the first time.
Strange notions such as the wave-particle duality
of matter and the theory of wave-mechanics
allowed for a model of the atom that made sense,
even if it was impossible to picture using the
common sense ideas based in traditional
understanding of the physical world. This new
model was so radical that it overturned much
conventional physics at the atomic level. Big
names in this development include Compton, de
Broglie, Schrödinger and Heisenberg.
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A Brief history of (models of) the
atom10 Post 1920
Experimental discoveries have caused atomic model
makers to modify and extend the basic quantum
model of the 1920s. The discoveries of
anti-particles, the neutron and a host of
so-called fundamental particles, led to new
theories of atoms and their constituents. Quantum
field theory was needed to talk about
interactions between particles. The realisation
that protons and neutrons were themselves not the
smallest indivisible particles (or waves!) has
resulted in the exotic physics of quarks and
theories of quantum chromodynamics alongside
quantum electrodynamics. All of these represent
developments of our model of the atom. Many of
these models are now highly mathematical and
impossible to represent in the diagrams familiar
to school science. Newton might be pleased to
note that God looks like he is indeed a
mathematician!

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So what might this tell us about models in
science?

• There is an underlying conviction amongst
  virtually all scientists that they are studying
  an independent reality. In this context, there is
  a belief that atoms are real. They are more than
  a useful device for controlling the world. In
  other words scientists are realists not
  instrumentalists. Indeed most reflective
  scientists are critical realists - they accept
  that their models of reality, however good, are
  subject to modification in the light of new
  knowledge. Our models and theories are
  necessarily provisional.