chemical elements and their compounds - Sciencemadness Dot Org

Monatomic Hydrogen 15
The Molecule-ion HJ
This ion occurs in positive ray tubes, 9 and also gives a characteristic
band spectrum. It is of great theoretical interest, as the simplest possible
case of two nuclei held together by a single electron: it contains the most
certain 'one-electron bond'. The physics of this link was first worked
out by Hund, and has since been discussed in more detail especially by
Pauling 10 and Dickinson 11 ; Pauling has given a general account in his
Chemical Bond, pp. 13-17, from which the following is derived. The formation
of such a link from a neutral hydrogen atom and a hydrogen ion
cannot be explained by classical mechanics, but only by wave mechanics
through the theory of resonance; the molecule is a combination of the
two states H+H + and H + +H; the bond may be said to owe its stability
to the resonance of the electron back and forth between the two nuclei,
with a resonance frequency equal to the resonance energy (50 k.cals./
mole) divided by the Planck constant h; this frequency is 7 X 10 14 per
second, whieh is about one-fifth as great as the frequency of orbital motion
about the nucleus, of the electron in. the normal hydrogen atom.
A more detailed investigation of the problem by Dickinson 11 and others 12
has shown that the total energy of the bond is 61 k.cals./mole, and the
internuclear distance 1*06 A, values which agree closely with experiment;
the values for the normal 2-electron link in H-H are 1034 k.cals. and
0-75 A.
Monatomic Hydrogen
At high temperatures hydrogen molecules are to some extent dissociated
into atoms, but on account of the large heat of dissociation (103*4 k.cals.)
the amount of atoms formed is very small, being at 1 atmosphere about
1 per cent, at 2,000° K. and about 9 per cent, at 3,000° K. 13
The molecules can, however, be dissociated by other means at lower
temperatures, and they recombine relatively slowly (owing to the difficulty
of getting rid of the heat of reaction*) especially at low pressures,
so that the gas can be more or less completely converted into single atoms,
which remain in that state long enough for their properties to be examined.
This is known as 'active' hydrogen, and it is probably the only active
form which exists. It can be prepared in various ways.
1. Langmuir 14 has shown that wires of tungsten, platinum, or palladium,
heated to temperatures of from 1,000° to 2,000° C. in hydrogen gas at
pressures of 0*01 mm. or less, give rise to atomic hydrogen (by absorbing
* Bee below, p. 17.
9 First observed by J. J. Thomson, Phil. Mag. 1911, [6] 21, 234, 239.
10 L. Pauling, Chem. Rev. 1928, 5f 173; of. B. N. Finkelstein and G. E. Horowitz,
JS, Phys. 1928, 48, 118.
» B. N. Dickinson, J. QUm. Phys. 1933, lf 317.
" E. A. Hylleraas, Z. Phys. 1931, 71, 739; G. JaffS, ib. 1934, 87, 535.
11 1. Langmuir, Gm. Electric Review, 1926, 29, 153.
" J.A.GJ5. 1912, 34, 1810 j 1914, 36» 1708; 1915, 37, 417; 1916, 38, 1145.