Theories of Atomic Structure

The Rutherford Model

Rutherford’s theory about the structure of atoms is known as the Rutherford model. Some aspects of the Rutherford model were discussed on the previous pages, but let’s examine the model more closely. According to Rutherford, atoms are comprised of electrons, protons, and (in most cases) neutrons. The protons and neutrons are responsible for nearly all of an atom’s mass, and are bundled together in a tiny nucleus at the center of the atom. Electrons are the outermost parts of the atom, with nothing but empty space between the electrons and the nucleus.

But electrons and protons have opposite electric charges, and therefore should attract each other. (Remember Coulomb’s law?) So, if there is nothing but empty space between the electrons and the nucleus, why doesn’t the atom collapse? What holds the electrons away from the nucleus? Rutherford hypothesized that each atom is like a miniature solar system, with the nucleus analogous to the sun and electrons analogous to planets. Planets orbit the sun, and thus don’t fall into the sun despite the sun’s gravitational pull. Perhaps electrons orbit the nucleus of an atom in a similar way, Rutherford suggested.

This was a clever hypothesis, but physicists quickly realized that it couldn’t be entirely right. Surely the orbits of electrons would be disturbed by electric fields from neighboring atoms, throwing the electrons out of orbit. Moreover, the Rutherford model seemed to violate Maxwell’s equations. As you may recall from chapter 2 (here), Maxwell’s equations imply that electromagnetic radiation is produced whenever a charged object is accelerated. Orbital motion involves acceleration: an orbiting electron constantly changes its direction of motion as it travels around the nucleus. Therefore, the electrons in an atom should constantly emit electromagnetic radiation. But they don’t! And even if they did, the energy carried by that radiation would have to come from somewhere; otherwise the first law of thermodynamics (conservation of energy) would be violated. Presumably the energy to produce the radiation would come from the kinetic energy of the moving electrons. But in that case, the electrons would slow down, and they would spiral inward toward the nucleus rather than maintaining stable orbits.