Atomic Structure

The topic of this article is more than a bit ironic, as the term atom derives from the Greek word atamos, which means something that cannot be further divided. Nevertheless, atoms do have components, and we will take a quick trip through the substructure of an atom.

To begin this story, an atom is the smallest entity that shares common chemical reactivity with one of the chemical elements, such as carbon or gold. For example, a lump of gold consists of a very large number of gold atoms. In fact, a gram of gold (about $37 worth at present) is made up of about 3 x 10^21 atoms – 3 billion trillion atoms. An individual gold atom usually has different physical properties than a lump of gold, because many such properties result from collective interactions of many atoms. For example, a gold atom is not a metallic solid; its color is not golden; it is not soft or easily deformable; its specific heat and thermal conductivity are, roughly speaking, undefined. Gold’s chemistry, however, is essentially the same for an atom as for a ton.

The first indication that atoms might have a substructure with components was Thomson’s 1897 discovery of the electron. Thomson experimented with cathode rays, or the electrons boiled from a hot filament and accelerated by their interaction with an electric field. He found that the electron had a negative charge and a mass less than a thousandth of that of a hydrogen atom, consistent with the notion that the electrons were a component of the atom.

Because chemical elements have no net electrical charge, the negative charge of the electrons in an atom must be offset by another atomic component having the same amount of positive charge to offset the charge of the electrons contained therein. This led to the plum pudding model of the atom, in which electrons are suspended in a uniform positively charged ‘pudding’ which supplies most of the mass of the atom.

The plum pudding model survived for barely a decade. In 1909, researchers in the laboratory of Ernest Rutherford bombarded a thin gold foil with alpha particles, which they had previously shown to be positively charged helium ions. They measured the angle through which the alpha particles were deflected by the foil, and discovered that the deflection angles were much greater than predicted by the plum pudding – if fact, some of the alpha particles were bounced directly back at the source. Rutherford’s comment on this observation: “as though you had fired a 16-inch shell at a sheet of tissue and it had bounced straight back and hit you!”. These experiments led to the Rutherford model of the atom, in which the nucleus, a body inside the atom more than 1000 times smaller than the atom, contained the positive charge and most of the atomic mass.

The particle that carries the positive charge of the atom is the proton, which was also discovered by Rutherford’s group in 1919. They discovered that when nitrogen gas was bombarded with alpha particles, hydrogen appeared in the previously hydrogen-free system. This demonstrated that the atomic nuclei must contain the hydrogen nucleus, a result that eventually identified the hydrogen nucleus as the second component of the atom, the particle that carries positive charge and much of the atomic mass.

The problem that remained was that protons were not heavy enough to reach the observed atomic mass by themselves. For example, Rutherford’s group had measured the charge of an alpha particle as twice that of an electron, which later suggested that it contains two protons. However, the mass of the helium atom is very nearly 4 times that of the hydrogen atom. There must be a third component of the atom, contained in the nucleus, having no electric charge but quite a bit of mass.

Rutherford suggested in 1920 that an atomic nucleus contains, in addition to protons, a new particle he called the neutron. His suggestion was not eagerly received at first, as an alpha particle could contain 4 protons and 2 electrons. However, this electron-proton model of the nucleus began to stumble in the wake of more delicate experiments.

Hints that atoms might have some property similar to the spin of a gyroscope surfaced regularly, arguably beginning with the discovery in 1896 of the Zeeman Effect, in which the light emitted from hot atoms is spread over more of the spectrum when a magnetic field is applied. The existence and nature of nuclear spin as “eine klassisch nicht beschreibbare Art von Zweideutigkeit” – a classically indescribable two-valuedness – was finalized in the 1920s by Pauli, Uhlenbeck, Goudsmit, Thomas, and others. Both electrons and protons have a spin of of the basic unit for angular momentum (which is quantized and restricted to integral multiples of this basic unit.

A puzzle in nuclear physics involved the spin of the nitrogen-14 nucleus. Experiments showed that it had a total angular momentum of 1 basic unit. However, within the electron-proton model of the atomic nucleus, this nucleus would have 14 protons and 7 electrons, each of which has a spin of . Under the rules of quantum mechanics, there is no way to put together 21 spin particles so that the total nuclear spin is one.

However, this observation is consistent with Rutherford’s neutron model, as long as the neutron has roughly the same mass as a proton and a spin of 1/2. Now the nitrogen-14 nucleus is made up of 6 proton-neutron pairs, arranged so that their spin is zero, and an additional proton and neutron, arranged so their spins add up to 1.

Indirect evidence for the existence of the neutron as a component of the atom was building. In 1932, James Chadwick used scattering experiments combined with the laws of conservation of energy and momentum to demonstrate that the nucleus contains neutrons. Chadwick’s result is considered to be the discovery of the neutron.

The electron, proton, and neutron are the basic components of all atoms. As a final comment, although the electron appears to be an elementary particle (having no substructure), this is not true of the other atomic components. The proton and neutron are each made up of three quarks, the proton of 2 up quarks and a down quark, and the neutron of an up quark and two down quarks. The quarks are tightly confined in high-momentum states within the nucleons.

It was long thought that the quark structure of protons and neutrons was unchanged in the atomic nucleus owing to the enormous difference in quark and nucleon binding energies. A clue that this is not quite true is that neutrons are stable inside a nucleus, but decay with a half life of about 15 minutes into a proton, an electron, and an electron antineutrino. In research reported only last month, scientists at Argonne National Laboratory have found that the momentum of quarks within nucleons in a nucleus is smaller than for the free nucleons, and that the amount of difference depends on the local environment of the quarks in the nucleus. This result is not well understood at this point, but does suggest that these sub-nuclear components of atoms may be important to the understanding of atoms.