The Atom


The Atom Essay, Research Paper

The Atom

AP Physics Period 2

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

lightest 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

remainder 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

elements 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

of 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

mistakenly 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

lines 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

physics 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

produced 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

events 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

world 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.


Rodes, Richard. The Making of the Atomic Bomb. New York: Ssimon and Schuster,


“Nuclear Wapon.” The Enclopedia Britannica. Encylopedia Britannica In.


V8; 1991, p 820-821.


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