Transcript A brief history of the model of the atom

A BRIEF HISTORY OF
MODELS OF THE ATOM
2b: Models in Science and Religion
A Brief history of (models of) the atom
[1] 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.
A Brief history of (models of) the atom
[2] 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 form’d Matter in solid,
massy, hard, impenetrable, moveable Particles…no ordinary
Power being able to divide what God himself made.”
A Brief history of (models of) the atom
[3] 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.
A Brief history of (models of) the atom
[4] 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 1850’s did caloric theory
collapse. Clausius, Boltzmann and Maxwell were able to
fully develop kinetic theory based on particles in motion.
A Brief history of (models of) the atom
[5] 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?
A Brief history of (models of) the atom
[6] 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.
A Brief history of (models of) the atom
[7] Rutherford’s model
Rutherford’s 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.
A Brief history of (models of) the atom
[8] 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 Bohr’s 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 1920’s
needed replacing.
A Brief history of (models of) the atom
[9] 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 wavemechanics 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.
A Brief history of (models of) the atom
[10] Post 1920
Experimental discoveries have caused atomic model makers to
modify and extend the basic quantum model of the 1920’s. 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.