March 25, 2006

The Nucleus of the Atom

Ernest Rutherford, The Scattering of α and β particles by matter and the structure of the atom, 1911

This one is really easy to understand, conceptually. Before this paper came along, scientists had a good idea of the size, mass, and electric charge of each type of atom. The then-current model of the atom had its mass and positive charge distributed evenly throughout its volume, with electrons embedded essentially at random. Electromagnetic forces were well understood, so collisions between charged particles could be accurately modelled.

Rutherford, working with Geiger, built some radiation guns (which fire the α and β particles referred to in the paper's title), aimed them at thin foils, and built detectors to measure deflections of the radiation particles caused by collisions with the atoms in the foils. The detectors had to be set up in particular arcs, so they looked in the expected places first, and found the expected deflections. Then Rutherford assigned a graduate student to look for deflections where there were none expected. To everyone's surprise, very large deflections were discovered. Some radiation actually bounced back from the atoms, a result Rutherford commented was "... as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you."

Rutherford's paper proposes a new model of the atom: practically all of its mass, and all of its positive charge, is concentrated in the center of the atom. If an atom were the size of a sports stadium, the nucleus would be about the size of a pea. Through this concentration, the paths of particles encountering the atom can be greatly perturbed, where they would pass through a diffuse atom almost unaffected. Rutherford demolishes the notion that the observed deflections could be caused by an accumulation of small encounters by working out the probabilities and noting that the observations show a much greater number of deflections than the old model would predict.

Rutherford also notes that the concentration of positive charge in the nucleus implies a vast amount of energy is preventing the charges from flying apart, and proposes that α particles are expelled from the nucleus of the atom. This naturally explains the high velocities of α particles when they are expelled from radioactive atoms.

So: the atom, previously thought to be an indivisible unit of matter, was found to have internal structure and smaller parts. That opens up a whole new can of worms: are these smaller parts also divisible? Is there, in fact, a smallest, indivisible piece of matter?


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