The inert pair effect: an analysis using the Chemdex database

The presence of a so-called ‘inert pair effect’ in the chemistry of the 6p elements has been recognised for almost a century. Following a brief historical overview and a summary of the explanations that have been advanced to account for this effect, a more quantitative study of its prevalence and importance in the chemistry of these elements is presented based on an analysis of data in the recently published Chemdex database. These data clearly reveal a preponderance of lower valent compounds for the elements thallium, lead and bismuth and demonstrate that the various compound types exhibiting lower or higher valances are in accord with the published explanations.