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Physical Structure
Main Concepts
A Bit of History

A Little Bit of History

In the early 1900's, if you were interested in atoms, you went to England. Sir J.J. Thompson in Cambridge had demonstrated that an atom could be broken up into smaller pieces, and in Manchester, Ernest Rutherford had a dynamic group working on atomic structure.

Niels Bohr was interested in atoms, so, he went to England.

Born in Copenhagen on Oct. 7, 1885, young Niels showed an early aptitude for science. His father was a professor of physiology at the University of Copenhagen, and his younger brother, Harald, was a genius with numbers and eventually became a brilliant mathematician. Between them, they encouraged the young Dane to excel in his school work and apply to the University of Copenhagen.

Bohr loved to think. His analysis of surface tension from a theoretical standpoint, and his measurements of vibrations in jets of water earned him a gold medal from the Royal Danish Academy of Sciences and Letters. Anything he could not see fascinated Bohr. Naturally he stayed on in science to get a Ph.D and wrote a doctoral thesis, all theoretical, about how electrons behaved in metals. Readers of this work were left in no doubt about Bohr's ideas on the inadequacies of current thinking on atomic structure.

The move to Manchester

So, he went to England, where all the action was taking place. He went first to the laboratory of Sir J.J. Thompson, in Cambridge. He expected a positive reception from this famous English scientist, since Thompson himself had worked on electrons in metals earlier in his career, but it was not to be. Somewhat disappointed, Bohr moved on to Manchester in March 1912 and joined the group working around an equally famous atomic scientist, Ernest Rutherford.

In those days Manchester was a black city. Coal fueled the industrial revolution and the soot and smoke had covered every building with a thick layer of black carbon. All buildings looked alike, and when the atmosphere inverted, smogs descended on the streets that made it impossible to walk along the sidewalk without bumping into the lampposts. But is was in Manchester that Bohr began to think about the atom again. Rutherford had recently proposed a new model for the atom, and his young, new, Danish colleague quickly took up the challenge of working on its theoretical implications.

Bohr spots a mistake

Bohr saw that something was wrong. Rutherford's atom had a small, massively dense nucleus (the "atomic center") that was surrounded by "orbiting" electrons. As far as Bohr could see, this arrangement was mechanically and electomagnetically unstable. The orbiting electrons would simply spiral down into the atomic center and vanish!

Unsatisfied with the answers given by classical physics, Bohr turned to the new ideas of quantum theory just being developed by Max Planck and Albert Einstein.

Although his own ideas were not fully worked out, Bohr suggested that Rutherford's picture of an atom was possible, but only if the electrons were restricted to a few discrete positions or states. The determining factor was the energy value defined by each state. At any one of these energy values, the atom was stable. In-between these values, the atom was unstable and could not exist.

Rutherford loved the idea.

But Bohr wasn't finished yet. He also saw that the various chemical properties of atoms depended on the electrons buzzing around the atomic center, and that, in turn, depended on the atomic number. Somehow, the natural units of electrical charge in the center of the atom, and the equivalent number of opposite charges in the electrons, made atoms chemically react the way they did.

When agitated in certain ways, atoms of hydrogen give off light; but not of all colors. The spectrum of light emitted by hydrogen atoms are a series of lines which look a bit like the bar codes used at a supermarket checkout.

Impressively, Bohr's quantum treatment of an atom's structure could explain this strange light pattern. Atoms in a stable state, according to Bohr, would not emit light. They would only do so as the electrons in the atom moved from one state to another. The frequency of the light (it's color) would depend on the difference in energy between these two states. Once again, electrons were the answer.

Electrons could only move around atoms in orbitals of fixed energy. The higher the orbital, the higher the energy, the lower the orbital the lower the energy. When an electron moved from a high energy orbital to a lower energy orbital it gave off the excess energy as light. Neon lights give off a yellowish color for this very reason.

A Marriage and a Thesis

Niels left Manchester and its smogs in the summer of 1912 and promptly got married. He continued his thinking, however, and published a complete treatment of his ideas in England in 1913. The world of atoms has never been the same since. Of course, Niels returned as a lecturer to England but eventually the University of Copenhagen lured him back with the offer of a Professorship, and then later created for him the Institute of Theoretical Physics, which opened its doors in 1921. The thinking Dane remained there as its director for the rest of his life, but he never forgot the soot covered buildings of Manchester where his key ideas had come to life.

© 2003, Professor John Blamire