HEAT OF FORMATION AND ELECTRON CONFIGURATION

A silver ion can be imagined to be formed from a potassium ion by adding ten positive units to the nucleus and surrounding the ion by ten more d electrons. If it is found by this process that the attraction of a negative charge at a distance r is increased, it follows that the effect of the positive charge, which is added to the nucleus, is not completely annihilated by the ten electrons or, in other words, the ten f electrons do not completely screen the ten positive units 

If this assumption is made, many more details of the heat of formation as a function of electron configuration can be readily explained. All examples will be taken from the group of the chlorides see Table XXIII), for which many data on heats of formation are known. However, the statements can be verified in other cases as well. The 3d10 shell is completed in the fourth period from Sc to Ni. In this group the heats of formation of many dichlorides have been measured. 

There are only few data on the trichlorides, but here again the same decrease is observed, and again there is the higher value for FeCl3, because of the half-filling of the 3d10 shell. A similar behaviour 

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