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. 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. 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 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 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 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.

A Brief history of (models of) the atom [10] Post 1920 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!

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.