A system of physical chemistry - Index of

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A system of physical chemistry - Index of

RUTHERFORD-BOHR ATOM-MODEL 117

atoms are approaching one another at a rate which is sufficiently slow

that the dynamical equilibrium of the electrons for every position of

the nuclei is the same as if the latter were at rest. Suppose that the

electron of atom i is rotating in the same plane as that of atom 2, the

difference in phase being one-half of a revolution. During the approach,

the direction of the planes of the orbits of the electrons and

the difference in phase will be unaltered. The planes of the orbits

will, however, at the beginning of the process approach each other at

a higher rate than do the nuclei, for two electrons rotating in the same

direction attract one another. Finally, at a certain distance of the

nuclei apart the electron planes will coincide, the two electrons being

now arranged in a single ring. During the further approach of the

nuclei the ratio between the diameter of the electron ring and the

distance apart of the nuclei will increase,^ and the system will pass

through a configuration in which it will be in equilibrium without the

application of extraneous forces on the nuclei. That is, a permanent

neutral hydrogen molecu'e is formed.

Now let us suppose that we are dealing with two helium atoms, i.e.

systems consisting of a nucleus of charge le surrounded by a ring of

two electrons, and let us imagine the above process repeated. Assume

that at the beginning the helium atoms are orientated to each other like

the hydrogen atoms, with the exception that the electrons in the helium

atoms differ by one-quarter of a revolution instead of one-half The

planes of the electrons will again approach one another at a higher rate

than do the nuclei, and for a certain position of the latter the planes

will coincide. During the further approach of the nuclei, the 4 electrons

will be arranged at equal angular intervals in a single ring. As

in the former case it may be shown that at any moment during this

operation the system will be stable for a displacement of the electrons

perpendicular to the plane of the ring. Contrary, however, to what

took place in the case of hydrogen, the extraneous forces to be applied

to the nuclei in order to keep the system in equiUbrium will always be

in a direction to diminish the distance apart of the nuclei, and the

system will never pass through a position of equilibrium. The helium

atoms will in fact repel one another and no molecule will be permanently

formed.

As regards molecular systems containing a considerable number of

electrons the treatment becomes much more complicated. Bohr

attempts to deal with such cases, and for details his paper must be

consulted. It may be mentioned that, following an approximate

method of calculation, Bohr is led to expect an absorption band in the

infra-red spectrum of hydrochloric acid gas at v = 13*7 x 10^^ whilst

the observed band lies at v = 8-5 x 10^^.

Bohr regards the water molecule as consisting of an oxygen nucleus

surrounded by a ring of four electrons, and the two hydrogen nuclei

situated on the axis of the ring at equal distances apart from the oxygen

' Of course the actual diameter of the electron ring diminishes but not so fast as

does the distance between the nuclei.

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