Periodic Table, The: Past, Present, And Future

and onward, the cumbersome term beyond superheavy elements was recommended.

Ephemeral Elements

Overlapping — but not identical to — the superheavy elements are the ephemeral elements [37]. The concept of ephemerality derives from the Greek word “ephemeros,” literally meaning “lasting only one day.” The term “ephemeral elements” is not widely used, but it is a useful concept to indicate elements for which there is no known isotope with a half-life of more than a day (hence, following the Greek meaning). In this way, we can roughly identify those elements for which the chemistry is either theoretical, or else based upon the study of individual atoms. At the time of writing, the ephemeral elements are those shown in Figure 5.9.

An isotope of dubnium, element 105, ²⁶⁸Db, has a half-life of 28 hours, thus dubnium is excluded from the ephemeral element list. This list may diminish if longer lived isotopes of any of these elements are synthesized. On the other hand, it will increase if elements beyond z = 118 are discovered. Confusingly, the term “ephemeral elements” has been used to describe claims for new elements that were subsequently shown to be false [38].

Figure 5.9 The ephemeral elements (at the date of writing).

Commentary

It is quite remarkable that well into the 21st century there are still disagreements on which elements fit into which categories — and the terminology. Hopefully, this chapter has provided some “food for thought.”

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Chapter 6

Isoelectronicity

Isoelectronicity is a concept widely used in chemistry — or often misused. It is difficult to think of a more ambiguous term. This chapter commences with a summary of the various ways in which isoelectronic has been used, together with the modifiers that have been applied to delineate its meaning in a specific context. After providing a consistent series of definitions, the richness of isoelectronic relationships, in all their forms, will be explored.

The term isoelectronic is ubiquitous in chemistry. But what precisely does a writer mean when