The Atom

In the spring of 1897 J. J. Thomson demonstrated that the beam of glowing matter in a cathode-ray tube was not made of light waves, as “the almost unanimous opinion of German physicists” held. Rather, cathode rays were negatively charged particles boiling off the negative cathode and attracted to the positive anode. These particles could be deflected by an electric field and bent into curved paths by a magnetic field. They were much lighter than hydrogen atoms and were identical “what ever the gas through which the discharge passes” if gas was introduced into the tube.

Since they were lighter than the ightest known kind of matter and identical regardless of the kind of matter they were born from, it followed that they must be some basic constituent part of matter, and if they were a part, then there must be a whole. The real, physical electron implied a real, physical atom: the particulate theory of matter was therefore justified for the first time convincingly by physical experiment. They sang success at the annual Cavendish dinner.

Armed with the electron, and knowing from other experiment that what was left when electrons were stripped away from an atom was much more massive emainder that was positively charged, Thomson went on in the next decade to develop a model of the atom that came to be called the “plum pudding” model. The Thomson atom, “a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification” like raisins in a pudding, was a hybrid: particulate electrons and diffuse remainder.

It served the useful purpose of demonstrating mathematically that electrons could be arranged in a stable configurations within an atom and that the mathematically stable arrangements could account for the similarities and regularities among chemical lements that the periodic table of the elements displays. It was becoming clear that the electrons were responsible for chemical affinities between elements, that chemistry was ultimately electrical. Thomson just missed discovering X rays in 1884.

He was not so unlucky in legend as the Oxford physicist Frederick Smith, who found that photographic plates kept near a cathode-ray tube were liable to be fogged and merely told his assistant to move them to another place. Thomson noticed that glass tubing held “at a distance of some feet from the discharge-tube” fluoresced just as the wall f the tube itself did when bombarded with cathode rays, but he was too intent on studying the rays themselves to purse the cause.

Rontgen isolated the effect by covering his cathode-ray tube with black paper. When a nearby screen of florescent material still glowed he realized that whatever was causing the screen to glow was passing through the paper and intervening with the air. If he held his hand between the covered tube and the screen, his hand slightly reduced the glow on the screen but in the dark shadow he could see his bones. Rontgen’s discovery intrigued other researchers beside J. J. Thomson and Ernest Rutherford.

The Frenchman Hernri Becquerel was a third-generation physicist who, like his father and grandfather before him, occupied the chair of physics at the Musee Historie in Pairs; like them also he was an expert on phosphorescence and fluorescence. In his case, particular of uranium. He heard a report of Rontgen’s work at the weekly meeting of the Academie des Sciences on January 20, 1896. He learned that the X rays emerged from the fluorescence glass, which immediately suggested to him that he should test various fluorescence materials to see if they also emitted X rays.

He worked for ten days without success, read an article on X rays in January 30 that encouraged him to keep working and decided to try a uranium slat, uranyl potassium sulfate. His first experiment succeeded-he found that the uranium salt emitted radiation but misled him. He had sealed a photographic plate in black paper, sprinkled a layer of uranium salt onto the paper and “exposed the whole thing to the sun for several hours. ” When he developed the photographic plate “I saw the silhouette of the phosphorescent substance in black on the negative. He istakenly thought sunlight activated the effect, much as a cathode ray releases Rontgen’s X rays from the glass.

The story of Becqueerel’s subsequent serendipity is famous. When he tried to repeat his experiment on Feb. 26 and again on February 27 Paris was covered with clouds. He put the uncovered photographic plate away in a dark drawer, with the uranium salt in place. On March 1 he decided to go ahead and develop the play, “expecting to find the images very feeble. On the contrary, the silhouettes appeared with great intensity.

I thought a t once that the action might be able to go on in the dark. Energetic, penetrating radiation from inert matter unstimulated by rays or light: now Rutherford had his subject, as Marie and Pierre Curie, looking for the pure element that radiated, had their backbreaking work. But no one understood what produced the lines. At best, mathematicians and spectroscopists who liked to play with wavelength numbers were able to find beautiful harmonic regularities among sets of spectral lines.

Johann Balmer, a nineteenth-century Swiss mathematical physicist, identified in 1885 one of the most basic harmonies, a formula for calculating the wavelengths of the spectral ines of hydrogen. these collectively called the Balmer series. It is not necessary to understand mathematics to appreciate the simplicity of the formula Balmer derived that predicts a line’s location on spectral bad to an accuracy of within on part in a thousand, a formula that has only on arbitrary number: lambdda=3646(n^2/n^2-4).

Using this formula, Balmer was able to predict the wavelengths of lines to be expected for parts of the hydrogen spectrum not yet studied. / They were found where he said they would be. Bohr would have known these formula and numbers from undergraduate hysics especially since Christensen was an admirer of Rydberg and had thoroughly studied his work. But spectroscopy was far from Bohr’s field and he presumably had forgotten them. He sought out his old friend and classmate, Hans Hansen, a physicists and student of spectroscopy just returned from Gottigen.

Hansen reviewed the regularity of line spectra with him. Bohr looked up the numbers. “As soon as I saw Balmer’s formula,” he said afterward, “the whole thing was immediately clear to me. ” What was immediately clear was the relationship between his orbiting electrons and the lines of spectral light. Bohr proposed that an electron bound to a nucleus normally occupies a stable, basic orbit called a ground state. Add energy to the atom, heat it for example, the electron responds by jumping to a higher orbit, one of the more energetic stationary states farther away from the nucleus.

Add more energy and the electron continues jumping to higher orbits. Cease adding energy-leaving the atom alone-and the electron jump back to their ground states. With each jump, each electron emits a photon of characteristic energy. The jumps, and so the photon energies , are limited by Plank’s constant. Subtract the value of a lower-energy stationary state W2 from the value of a higher energy stationary state W1 and you can get exactly the energy of light as hv.

So here was the physical mechanisms of Plank’s cavity radiation. From this elegant simplification, W1-W2=hv, Bohr was able to derive the Balmer series. The lines of the Balmer series turn out to be exactly the energies of the photons that the hydrogen electron emits when it jumps down from orbit to orbit to its ground state. Then, sensationally, with the simple formula, R=2pi^2me^4/h^3, Bolar roduced Rydberg’s constant, calculation it within 7 percent of its experimentally measured value. There is nothing in the world which impresses a physicist more,” an American physicist comments, “than a numerical agreement between experiment and theory, and I do not think that there can ever have been a numerical agreement more impressive than this one, as I can testify who remember its advent. ”

“On the constitution of atoms and molecules” was seminally important to physics. Bexzides proposing a useful model for the atom, it demonstrated that vents ensts that take place on the atomic scale are quantized: that just as matter exits as atoms and particle s in a state of essential graininess, so also does process.

Process is discontinuous and the “granule” of mechanistic physics was therefore imprecise; though a good approximation that worked for large-scale events, it failed to account for atomic subtleties. Bohr was happy to force this confrontation between the old physics and the new. He felt that it would be fruitful for physics. because original work is inherently rebellious, his paper was not only an examination of the physical orld but also a political document.

It proposed, in a sense, to begin a reform movement in physics: to limit claims and clear up epistemological fallacies. Mechanistic physics had become authoritarian. It had outreached itself to claim universal application, to claim that the universe and everything in it is rigidly governed by mechanistic cause and effect. That was Haeckelism carried to a cold extreme. It stifled Neils Bohr as a biological Haeckelism and stifled Christian Bohr and as a similar authoritarianism in philosophy and in bourgeois Christianty had stifled Soren Kierkegaard.


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