A system of physical chemistry - Index of

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A system of physical chemistry - Index of

RUTHERFORD-BOHR ATOM-MODEL 115

divides the line connecting the nuclei. If we consider a neutral system,

containing two nuclei carrying large charges, in the stable configuration

the greater part of the electrons must be arranged round each nucleus,

approximately, as if the other nucleus were absent ; and only a few of

the outer electrons will be arranged differently, rotating in a ring round

the line connecting the nuclei. This ring which keeps the system

together represents the chemical bond.

Systems Containing Few Electrons ; the Hydrogen Molecule.

As already stated, the neutral hydrogen molecule consists of two

similar nuclei each carrying unit charge e with two electrons rotating in

a ring between the nuclei. Denoting the radius of the ring by a, and

the distance apart of each nucleus from the plane of the ring by b, we

get from equation (8), on putting N = i, and n = 2—

From (11) we get further—

Fo = ^^~

3 '

= 1-049.

From (9) and (10) we get— denoting the values of a, w, and W for a.

system consisting of a single electron and single nucleus {i.e. the

hydrogen atom) by ag, a>o, and Wo—the following values for the hydrogen

molecule :— a = o*95ao; w = I'low^, ; W = 2-2oWo.

Since W is greater than 2W0, it follows that two hydrogen atoms

combine to form a molecule with emission of energy. That is a

hydrogen molecule dissociates with absorption of energy. This is in

agreement with -experiment. Putting Wo = 2*0 x io~^^ erg, and

No = 6'2 X 10-^, where No is the number of molecules in one grammolecule,

we get for the energy emitted during the formation ot one

gram-molecule of hydrogen from two gram-atoms, the quantity

No(W - 2 Wo) = 2-5 X 10^- erg, or 60,000 cals. This value is of the

correct order of magnitude, the value observed by Langmuir being

approximately 80,000 cals.

In considering displacements of electrons perpendicular to the plane

of the ring, we have seen that the molecule will become unstable unless

when N = i,«=2or3. Hence if we remove one of the two electrons

from the hydrogen molecule the whole system will become unstable

and will break up into a single nucleus positively charged and

a neutral hydrogen atom. This process corresponds to the change

from a stationary state represented by the complete neutral molecule

and a second stationary state represented by the dissociated molecule.

On Bohr's theory when such a change takes place from one stationary

to another there is involved homogeneous radiation of frequency v, the

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