A system of physical chemistry - Index of

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A system of physical chemistry - Index of

CHAPTER V.

(Physical equilibrium, continued) — Distribution of electrons in atoms—Structure

of the atom from the standpoint of the quantum theory —The Rutherford-

Bohr atom-model— Parson's magneton and the structure of the atom— High

frequency spectra of the elements— Moseley's relation.

T/ie Rutherford Atom-model.

(Rutherford, Phil. Mag., \y\.\ 21, 669 (191 1).}

The underlying idea in this atom-model is that the atom consists of a

central charge, concentrated on a nucleus, which is surrounded by an

" "

atmosphere of electrons, rotating in certain orbits. Practically the

whole mass of the atom is to be ascribed to the nucleus. The number

"

of "atmosphere electrons is not large. The dimensions of the nucleus

are considerably smaller than the dimensions of the atom as a whole.

Rutherford estimates the diameter of the nucleus to be of the order

io~^2 cm., whilst the diameter of the atom as a whole is of the order

io~^ cm. The nucleus itself possesses a structure about which, however,

nothing is known. It, the nucleus, contains probably quite a

large number of electrons, but these are bound firmly, except in the

case of radioactive materials which are capable of expelling one of these

bound electrons in the form of y8-rays. The outermost ring of the

"atmosphere'' electrons contains those which give the property of

valency to the atom. The innermost electron of the atmosphere is the

source of X-rays, the shortest wave-length which the atom is capable

of emitting. The necessary disturbance of the innermost electron is

brought about by collision with a ^-ray (cathode ray) which jerks the

innermost electron out ; on its return journey to the innermost ring it

causes the emission of the extremely short waves which we call X-rays.

Rutherford was led to this planetary view of the atom in order to ex-

plain the scattering of a and /3 particles.

very

When such particles pass

close to the centre of an atom they are violently defiected, an

effect which could be produced by a nucleus of the kind referred to.

The angle of scattering has been measured by Geiger and Marsden in

the case of a- rays, and by Crowther in the case of /t?-rays. The outer

"atmosphere" of electrons rotating in rings do not appreciably affect

the direction of the motion of an a particle travelling through the atom.

It is only when the a particle comes into close contact with the nucleus

that its path is abruptly altered, the path becoming hyperbolic.

96

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