A system of physical chemistry - Index of

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A system of physical chemistry - Index of

24

A SYSTEM OF PHYSICAL CHEMISTRY

In the case of a diatomic molecule we have therefore three degrees

of freedom in respect of translation, one degree of freedom in respect

of linear vibration of the atoms, and two degrees of freedom in respect

of molecular rotation ; six degrees in all. If we regard rotation as impossible

we have to substitute precessional vibration of the molecule

which corresponds to two degrees of freedom ; again six degrees of

freedom in all.

In the case of a molecule containing more than two atoms,

if rotation

be possible at all, it is reasonable to ascribe three degrees of

freedom in respect of such rotation, i.e. the limiting number of degrees

of freedom. At the same time the molecule possesses three degrees of

freedom in respect of translation and three degrees of freedom in respect

of atomic vibrations in pairs ; in all nine degrees of freedom. In the

case of translation and of rotation ^ the energy is entirely kinetic ; in

the case of vibration it is potential as well as kinetic. The possible

degrees of freedom in various kinds of gas molecule are summarised in

the following table which is due to Bjerrum [Zeitsch. Elektrochem., 17,

731, 1911) ; the general correctness of the assigned values is borne out

to a certain extent by the values of the molecular heats of the respective

gases :-

No. of Atoms in

the Molecule.

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