Mean arrival time, definition

It emerges that the notion that element 72 was not a rare earth was the commonly held view among a number of chemists. Element 72, or at least the vacant space that it was supposed to fill, was often placed beyond the rare earth block in published periodic tables prior to Bohr s theory, for example, by Bury. In fact, the prediction that hafnium is not a rare earth element can be obtained quite simply by counting and is by no means dependent on assuming the existence of electron shells. This can be illustrated as follows It had been known for some time that the number of elements in each period follows a definite sequence given by 2,8,8,18, 18, 32 (probably followed by 32), and so on. By adding the first six of these numbers, one arrives at the conclusion that the sixth period terminates with a noble gas of atomic number 86. It is a simple matter to work backward from this number to discover that element 72 should be a transition metal and a homologue of zirconium, which shows a valence of 4. This procedure depends on the plausible assumption that the third transition series should consist of 10 elements, as do the first and second transition series ... 

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