Wednesday, 16 December 2009

Within the atom; a popular view of electrons and quanta



"Now imagine^ if you can, two types of particles, each invisible, intangible and infinitesimal in the
ordinary senses of these words, and indeterminate in form and substance. For one tjrpe, wonderfully enough, we know the name "electron/' but for the other type there is no agreement. We are free to choose from a number advanced by various scientists and shall arbitrarily adopt the term "proton."

Electron and proton are complementary. Together they may merge in a union so close that their combined size is less than that of the electron alone. Such a statement may sound absurd but experiments seem to indicate that ihe union of two or more protons with one or more electrons is a smaller particle than is a single isolated electron. The form and size of the electron and proton must then be different in combination from that of the free electron and free proton respectively.

The protons and electrons are complementary, mutually supplying each other's needs. Electrons, however, are mutually antagonistic and depart from each other's presence unless restrained. The same is true of protons. It is only by virtue of the complementary properties of proton and electron that two or more electrons, for example, are constrained to the same infinitesimal space.

According to some theories^ however, two electrons or two protons are pictured as mutually attracted when they are very dose together, although at larger separations they are repellent. Similarly an electron and a proton would start to repel each other after they had approached to within a certain small distance of each other. In any case the permanence of a group of protons and electrons will depend upon the geometrical arrangement.

Electrons pellate, protons pellate, but an electron and a proton tractate.

Measurements of the tiny wave lengths involved in X-rays are, therefore, made possible by the use of crystals for which the dimensions of the "lattices" are known. The frequencies corresponding are then obtainable by simple arithmetic.

In sudi measurements the crystal is merely a portion of the instrument and there is no further concern with the physical mechanism whereby it operates. Such was the use to which Moseley put crystals in his famous investigations of 1914 before his life was sacrificed to a World War. He used the crystal grating which we have described above for the determination of the characteristic X-ray frequencies of various substances. The oscillators of the crystal will respond to radiations of a wide range of X-ray frequencies and re-radiate the same frequency as that with which they are excited.

Moseley took photographs successively of the X-radiation from various types of anti-cathodes.

Apparently each element of the periodic series differs from the next lower by the addition of a definite amount of electricity which is accompaoied by an increase in frequ^icy of the characteristic radiation. It is the nuclear charge which increases and thus gives rise to greats restoring forces and more rapid vibrations when the inner electrons are displaced.

Moseley's discovery of a simple numerical relationship between charactmstic frequencies did not involve measurements on all the known elements. Below aodium^ for example, there are ten elements for which no X-ray spectra have yet been obtained. The inert elements also must of necessity be omitted. Thus you will notice that krypton (atomic number 36) is omitted in Fig. 25. His work and conclusions, have been corroborated by many other tests and may be considered the first definite proof of the structure of the atomic nucleus by grains of positive electricity (protons).

Two phenomena are observable when X-rays impinge upon a substance. X-rays are re-radiated and electrons are ejected.

Just as substances exposed to X-rays give off their own characteristic vibrations when these are of lower frequency, so fluorescent substances when exposed to the invisible ultra-violet radiations will give off visible radiations. Electric arc-lights are quite rich in ultra-violet radiations, so that fluorescent sub- stances exposed to such light will glow with their characteristic radiations.

The electrons which are ejected when an X-ray passes through a substance start off with speeds and energies like those of the cathode rays whidh originated the radiation. As they pass through the substance they disturb other electrons and hence ionize large nxunbers of the atoms.

The phenomenon of the ejection of electrons, when the exciting radiation is ultra-violet or lies within the visible range, is known as the photo-electric effect. It promises to be of wide scientific interest, for it is apparently the cause of photo-chemical effects like those utilized in photography, of photo-synthesis in the formation of carbohydrates in plant life, and even of the effect of light on the retina of the eye.

The moment a substance is exposed to light of the proper frequency the photo-electric emission b^ins. This would appear to indicate that Hiere was a hopperful of energy in some electronic system which was tripped off, as by a trigger, and allowed to discharge. The energy which is released was either obtained from the beam of light, despite the short time of exposure, or was already stored in the atomic system. The further fact that the energy of the emitted electron is the same whether the intensity of the light is large or small would seem to indicate such a storage.

...When a body radiated energy it would really be shooting out in all directions a shower of invisible particles, small bundles of energy. The electron must then receive or reject a whole bundle. The picture of the ejection of electrons by X-rays which was quoted on page 140 would be ejcplained if the X-rays were really small bullets of energy which followed radial paths outward from the anti-cathode. What appears to us as a continuous distribution of energy in a wave is probably not really continuous but conforms in analogy to a fine shower of rain such aa one experiences when a fog blows m.

When an electric current is passed through a gas light is emitted.^ By using a spectrometer or a grating, involving the principles of interference which have been mentioned in previous chapters, this light may be analysed into a series of spectral lines, similar to but more numerous than those appearing in an X-ray spectrum. It is found that any element produces a spectrum in which lines recur at intervals throughout a given frequency range. These lines form a series, the frequency of each member of which may be calculated from that of the highest frequency by very simple arithmetic. In the case of incandescent hydrogen three such series are known: one in the visible range of frequency called the Balmer series; one in the region of lower frequency, the infra-red region, which is known as the Paschen series; and the third, known as the Lyman series, in the ultra-violet.

When the disturbance is excited by impacts, as in the case of "white'' X-rays, the highest frequency which is radiated is determined by the quantum of energy which is brought to the radiating substance by an impinging electron. A quantum relationship is also involved in radiations of lower frequency.

Let us suppose, however, that a radiating body is placed in an enclosure, as that of Fig. 33. Let the body be in equilibrium with the walls of its enclosure, that is with its surroundings, receiving from them by radiation just as much energy as it in turn is radiating to them. If either partner in this exchange were to absorb more radiation than it emitted its temperature would rise.

Suppose we construct a vibrating system by connecting a number of corks together by elastic bands. Imagine a complicated system, if you will, with a large number of cross connections between various corks. Now disturb this by pulling some of the corks from their equilibrium positions and then allow the natural oscillations to occur. Let this system with several different oscillations be placed on water. The corks simulate a vibrating system. The water, with its almost infinite niunber of tiny molecules, and hence mfinite possibilities for forms of vibration, simulates the ether. We know what happens.

To a very large extent, as we shall see, Planck's theory constituted a theory of probability for electrical oscillators.

As you remember, he assumed that an oscillator could handle only a quantum of energy; and by quantum he meant an amount proportional to the frequency of vibration, the amount hn. Oscillators of low frequency, even if relatively numerous, will handle but a small portion of the total energy and contribute but little because the amount which each individual oscillator may handle is small. On the other hand, oscillators of large frequency will respond only if there is available a relatively large amount of energy since their quanta are greater. To function, however, the higher frequency oscillator must receive its quantum all at once; it cannot make it up from several successive smaller quanta. Since large quanta will probably occur only infrequently, this requirement means that there will be little total energy associated with the oscillators of higji frequency. The maximum radiation, therefore, will occur in the middle range of frequencies, as the experimental sulta indicate.

You will remember that reflection is really re-radiation. Any reflected radiation must then include most prominently those radiations which are of the same frequency as the oscillators would themselves naturally emit. The phenomenon is one of resonance, so-called — ^that is the phenomenon of greatest response when the appeal strikes the proper personal note."

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