650 The Philippine Journal of Science 1921 
traction so that in the shells of the inert atoms they are in direct 
contact with each other. 12 In the shells of the other atoms, how- 
ever, it seems that the nuclear attraction is insufficient for such 
close union, and they separate, forming a larger, looser shell, 
but still have a strong tendency to form pairs. In the case of 
unshared electrons this tendency may not be effective, but when 
external influences concentrate electrons toward certain portions 
of the shell, pairs are formed. 
/ 
— > 
Fig. 4. Unsymmetrical weak union; firm electron. 
Forces uniting atoms . — If we consider the forces represented 
in fig’s. 2, 3, and 4 to be only the components of constraint in 
a direction parallel to the line joining nuclei D and C, then the 
intercept on the “equilibrium position” line of either “single 
constraint” line is the force with which the electron in question 
unites the two atoms. Considering the atoms to be at equilibrium 
distance, the binding forces of this and any other electrons 
which C and D may be sharing are exactly neutralized by other 
forces, which may be considered as one force; namely, the 
mutual repulsion of the two atoms. Therefore the strength of 
union is measured, not by the amount of this binding force, but 
12 Cf. Perkins, G. A., Philip. Journ. Sci. 18 (1921) 325, 19 (1921) 21. 
