Kinetic Theory : Shapes Of Molecules

The molecular diameter, σ, is calculated in various ways of which probably the most reliable is from the viscosity of the gas Usually the assumption is made that the molecule is spherical, and although this may be approximately true for diatomic molecules it may not be permissible for rod-shaped molecules, such as carbon dioxide and nitrous oxide, for which a single diameter can have very little meaning. More recently, attempts have been made to determine the distances between the atomic centres in such molecules, and these will obviously give a better picture of the structure of the molecule than a fictitious diameter.

The following table contains the values of the molecular diameters, σ in A.U., on the assumption of spherical molecules, as calculated from the viscosities.

He 1.88
Ne 2.36
Kr 3.23
Xe 3.54
H₂ 2.39
O₂ 2.96
N₂ 3.13
CO 3.22
CO₂ 3.39
N₂O 3.32
NO 2.9
HCl 2.6
H₂O 2.6
Cl₂ (1.60) 3.6
Br₂ 1.71
I₂ 1.88

The distances between the lines in band spectra depend on the moments of inertia of the molecules, hence these and from them the distances between the atoms may be calculated. In this way and from X-ray measurements it is found that in the gaseous and also in the solid state, the molecules of CO₂ and N₂O are rod-shaped. The H₂O molecule is a triangle with O at the apex; the NH₃ molecule is a flat tetrahedron with N at one apex and H at each of the other three. Methane is tetrahedral.

Protein crystallography
Chemistry guide
Kinetic Theory
-S-
<<<	>>>