A system of physical chemistry - Index of

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A system of physical chemistry - Index of

RUTHERFORD-BOHR ATOM-MODEL 97

Bohr's Application of the Quantum Theory to t/ie Rutherford Atom-model.

(Bohr, Phil. Mag., [vi.], 26, i., 476, 857 (1913) ; ibid., 27, 506 (1914) ;

30, 394 (1915);)

The following is a brief account of the line of thought pursued by

Bohr, given to a large extent in his own words.

Bohr first of all points out that the Rutherford atom-model, which

has just been referred to, suffers from the serious drawback that the

system of "atmosphere" electrons is unstable; that is, unstable from

the standpoint of classical electro-dynamics. If, however, we introduce

Planck's concept, the instability may no longer exist from the theoretical

point of view. The problem of atomic structure affords therefore a

further instance of the necessity of introducing some new concept, such

as that of Planck, into electro-dynamics in order to account for the

observed facts. Bohr first attempts to apply the quantum theory to

the process whereby a free electron— such for example as exists in

a vacuum tube when a discharge is passed—

may be conceived of as

attaching itself to a positively charged nucleus. It will be shown

that it is possible from the point of view taken to account, in a simple

way, for the Balmer law of linei spectra of hydrogen and helium, and

possibly the theory will eventually be capable of dealing with the spectra

of more complicated atomic structures.

According to Rutherford, the hydrogen atom consists of a nucleus

with a single electron describing a closed orbit around it. The first assumption

is that the mass of the electron is negligible compared with the

mass of the nucleus, and that the velocity of the electron is small compared

with that of light. Bohr considers that whilst the electron is rotatmg m

this orbit or stationary state it is neither radiating or absorbing energy.

Such rotation represents an equilibrium condition of the system as a

whole. The production of spectra must be due therefore to some kind

of departure from an equilibrium state. In general Bohr considers the

possibility of a series of such stationary states corresponding to electron

orbits of different radius, and emission or absorption of radiation is due

to the electron passingfrom, one stationary or equilibrium state to another.

The quantum idea is introduced in the further hypothesis that during

the passage of the electron from one equilibrium state to another,

homogeneous radiation of a certain frequency v is emitted or absorbed,

the amount of radiant energy so emitted or absorbed being hv, where

h is Planck's constant. It will be realised at once how very different

these considerations are from those based on classical electrodynamics.

On the classical view a charged particle rotating steadily in a closed

orbit is giving off radiation continuously, and in so doing its orbit

gradually becomes smaller so that the electron would finally drop into

the nucleus. Furthermore, with the alteration in orbit the wave-length

of the light emitted would gradually change, a conclusion which is opposed

to direct observation, for spectra are clearly characteristic and

unchangeable properties of an atom. Bohr assumes that no radiation

at all is given out when the electron is rotating in an orbit.

VOL. III.

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7

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