Five simple molecular potential functions have been tested by comparison with spectroscopic data for the hydrogen halides. It is shown that when plotted on a ``reduced'' distance scale (r/re), the potential curves for these molecules coincide closely up to Δ(r) = 0.10re. Two electrostatic functions indicate β/rn as the more satisfactory form of the repulsive term. Significant values of the effective polarizability and ``fractional ionic character'' are deduced. The latter are higher and more nearly constant than the uncorrected values currently quoted as showing changing bond type in these molecules. The results conform with Kirkwood's very successful treatment of HCl as a polarizable ion pair.

Of purely empirical functions, the Pöschl‐Teller provides a slight improvement upon the simple Morse relation, and is only a little more cumbersome than the latter. The Rosen‐Morse function is less satisfactory for these molecules. Simple expressions are given for evaluating the three parameters in these functions from spectroscopic data.

1.
L. Pauling, The Nature of the Chemical Bond (Cornell University Press, Ithaca 1939), p. 46;
C. P. Smyth, Frontiers in Chemistry, Vol. 5 (Interscience Publishers, Inc., New York, 1948), p. 31.
2.
See, for instance, Jevons, Report on Band Spectra of Diatomic Molecules (Physical Society, London, 1932), p. 27.
3.
K.
Fajans
,
Zeits. f. Physik. Chemie
B24
,
118
(
1934
);
O. K. Rice, Electronic Structure and Chemical Binding (McGraw‐Hill Book Company, Inc., New York, 1940), p. 178.
4.
G. Herzberg, Molecular Spectra and Molecular Structure: Vol. I: Diatomic Molecules (Prentice‐Hall, Inc., New York, 1939), p. 482 et seq.
5.
Nature
161
,
731
(
1948
). The correction arises from the nongaseous standard state to which the thermal data for Br2, and I2 are referred (Gaydon, private communication).
6.
A. S.
Coolidge
,
H. M.
James
, and
E. L.
Vernon
,
Phys. Rev.
54
,
726
(
1938
).
7.
M.
Born
and
W.
Heisenberg
,
Zeits. f. Physik
23
,
388
(
1924
).
8.
J. G.
Kirkwood
,
Physik. Zeits.
33
,
259
(
1932
).
9.
G.
Briegleb
,
Zeits. f. Physik. Chemie
B51
,
9
(
1941
).
10.
L.
Pauling
,
J. Am. Chem. Soc.
54
,
998
(
1932
).
11.
See reference 1, L. Pauling, p. 326.
12.
P. Debye, Polar Molecules (The Chemical Catalogue Company, New York, 1929), p. 62.
13.
See reference 1, Pauling and Smyth.
14.
Mansel
Davies
,
J. Chem. Phys.
15
,
739
(
1947
).
15.
P. M.
Morse
,
Phys. Rev.
34
,
57
(
1929
).
16.
N.
Rosen
and
P. M.
Morse
,
Phys. Rev.
42
,
210
(
1932
).
17.
R. de L. Kronig, The Optical Basis of the Theory of Valency (Cambridge University Press, Cambridge, England, 1935), p. 88.
18.
W.
Lotmar
,
Zeits. f. Physik
93
,
528
(
1935
).
19.
G.
Pöschl
and
E.
Teller
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Zeits. f. Physik
83
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143
(
1933
).
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