A system of physical chemistry - Index of

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A system of physical chemistry - Index of

FARSOiYS A TOM-MODEL 1 1 9

somewhat less than the accepted value for the diameter of an atom.

Parson's theory of atomic structure based upon the magneton hypothesis

may be summarised as follows :—

The positive charge in the neutral atom, which must be present to

balance the negative charges of the magnetons is regarded as a sphere

of uniform positive electrification possessing a volume which is proportional

to the number of magnetons which it contains. That is, the

distribution of the positive charge is once more treated as in the

original theory of atomic structure first advanced by Sir J. J. Thomson

{cf. Chap. I., Vol. I.). This positive sphere is regarded as having the

properties of an elastic solid and " is surrounded by an atmosphere or

envelope of very low charge density which is also elastic ". Given this

distribution, Parson shows that a group of eight magnetons may be

arranged symmetrically round the sphere so as to give a stable configuration;

and even with atoms possessing a larger number of magnetons,

a similar distribution of eight is assumed. Such an arrangement ob-

viously recalls the Abegg-Bodlander theory of valency, according to

which the natural number of valencies is eight, though all these are not

effective as far as another atom is concerned. To explain the existence

of the long periods in the period classification as well as the properties

of certain elements contained therein, it is necessary, to assume a certain

" hindrance " to the formation of the normal number of eight magnetons.

On the basis of these assumptions Parson discusses the single magnetic

attraction between two such magnetons, the tendency to form the

group of eight, the residual valency effect and the electrical polarisation

set up by a magneton of the above type. On the basis of this atomic

structure Parson has made a rough calculation of the heat of dissocia-

tion of hydrogen, obtaining the value 135,000 cals. per gram- molecule;

Langmuir's value being of the order 80,000 cals.

One of the most obvious advantages of the Parson magneton is that

it affords' a rational basis for the assumption made by Bohr, viz. that

an electron rotating in a stable orbit does not radiate. A rotating

electron— as hitherto envisaged — suffers an effective inward acceleration

in rotating, and hence, on the basis of the classical electromagnetic

theory, it should emit radiation continuously, its orbit becoming smaller

until it falls into the centre of the atom. On the basis of the classical

the number of

electromagnetic theory it is also known that the greater

electrons rotating in a ring the less the radiation from the ring. The

Parson is ring magneton equivalent to a continuous series of charges,

and consequently such a ring, howsoever great its velocity,

will not

radiate at all. This carries out Bohr's idea of the absence of radiation

from an electron in an orbit.

On the other hand one of the most obvious disadvantages of Parson's

theory of atomic structure lies in the assumption of a uniform positive

sphere of electricity, for on such a basis it is quite impossible to account

for the large angle scattering of a rays observed by Rutherford. The

point is therefore whether we can retain the advantage of the ring

electron or magneton and at the same time get rid of the hypothesis of

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