SUMMARY

Technetium ( Z=43) and promethium ( Z=61) are the only elements below bismuth ( Z=83) in atomic number that have no stable isotopes. The discovery of these unusual elements is described, and the physical factors leading to their instability is discussed.

1. INTRODUCTION

The last of the naturally occurring stable elements were discovered in 1923 ( ₇₂Hf) and 1925 ( ₇₅Re). This concluded over a century of rapid discovery of elements found in the earth s crust. Two holes among atomic numbers below ₈₃Bi remained: 43 and 61. Moseley s correlation of atomic number with the energies of transitions involving inner atomic electrons made it clear that the gaps were real.

The steps and missteps in the process of discovering new elements led to caution in accepting a previously unidentified spectroscopic feature as evidence for a new element. Chemical characterization was required. For example, masurium was proposed for element 43, and illium and florentium were proposed for element 61 based on atomic spectroscopy of extracts from minerals. In retrospect it is clear that there was evidence for unusual conditions for these elements. For example, above ₇N the only mass numbers of stable isotopes of elements with odd atomic number are also odd, and there is only one stable isobar for each odd A. Molybdenum has stable isotopes 92, 94 98, and 100. Ruthenium has 96, 98 102, and 104. Niobium has 93, and rhodium 103. Nothing is...