DIMENSIONS OF ATOMS AND MOLECULES 53 



It is possible to calculate the average area of the target 

 presented by such a molecule to other molecules approaching it 

 from all directions. When the molecule is end on, the target 

 will be no larger than that presented by a single atom, but 

 when its longer axis is perpendicular to the direction from which 

 the other molecule is approaching the target presented will be 

 nearly twice as great. Rankine has made calculations of these 

 areas, and in the following table they are compared with those 

 deduced from the viscosity of the gases. 



In this table the area tto-* ofthe inert gas atom is given in 

 the second column. The area A in the third column is that 

 calculated by Rankine for two atoms of corresponding dimen- 

 sions joined together by holding electrons in common, the 

 centres being separated by the appropriate distance deduced 

 from molecules in crystals. The area S in the fifth column is 

 that deduced from the viscosity of the corresponding diatomic 

 gas. In the last column is given the ratio of the calculated 

 and observed areas. The relation between the dimensions of 

 the chlorine molecule and argon atom, which has been already 

 referred to, would also be expected to exist between fluorine 

 and neon, bromine and krypton, iodine and xenon. Since the 

 viscosity of fluorine is not known, Rankine has compared the 

 oxygen molecule with the neon atom in this case. It will be 

 seen that the ratio in the last column is very nearly unity — a 

 fact which confirms the estimate which has been made as 

 to the distance separating the atomic centres. Rankine has 

 further applied the same reasoning in the case of the CO2 and 

 N2O molecules, which he has assumed to consist of three neon 

 structures linked together in a row, and again gets satisfactory 

 agreement between calculated and experimental values. 



Another method of estimating the distances between the 

 centres of atoms in a molecule is afforded by the absorption 

 spectra of gases. A gas such as HCl has an absorption band in 

 the infra-red, whose frequency, to a first approximation, corre- 

 sponds with the frequency of the radial vibration of the hydrogen 

 and chlorine atoms, towards and away from each other, under 

 the influence of the forces binding them together. When this 

 band is analysed it is found to consist of a number of regularly 

 spaced narrow bands. Owing to collisions the molecules of the 



