Period 7 element

A period 7 element is one of the chemical elements in the seventh row (or period) of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behavior of the elements as their atomic number increases: a new row is begun when chemical behavior begins to repeat, meaning that elements with similar behavior fall into the same vertical columns. The seventh period contains 32 elements, tied for the most with period 6, beginning with francium and ending with oganesson, the heaviest element currently discovered. As a rule, period 7 elements fill their 7s shells first, then their 5f, 6d, and 7p shells in that order, but there are exceptions, such as uranium.

All period 7 elements are radioactive. This period contains the actinides, which include plutonium, the last naturally occurring element; subsequent elements must be created artificially. While the first five of these synthetic elements (americium through einsteinium) are now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. The later transactinide elements have only been identified in laboratories in batches of a few atoms at a time.

Though the rarity of many of these elements means that experimental results are not many, their periodic and group trends are less well defined than other periods. Whilst francium and radium do show typical properties of their respective groups, actinides display a much greater variety of behavior and oxidation states than the lanthanides. These peculiarities are due to a variety of factors, including a large degree of spin–orbit coupling and relativistic effects, ultimately caused by the very high electric charge of their massive nuclei. Periodicity mostly holds throughout the 6d series and is predicted also for moscovium and livermorium, but the other four 7p elements, nihonium, flerovium, tennessine, and oganesson, are predicted to have very different properties from those expected for their groups.

(?) Prediction

(*) Exception to the Madelung rule.

In many periodic tables, the f-block is erroneously shifted one element to the right, so that lanthanum and actinium become d-block elements, and Ce–Lu and Th–Lr form the f-block tearing the d-block into two very uneven portions. This is a holdover from early erroneous measurements of electron configurations. Lev Landau and Evgeny Lifshitz pointed out in 1948 that lutetium is not an f-block element, and since then physical, chemical, and electronic evidence has overwhelmingly supported that the f-block contains the elements La–Yb and Ac–No, as shown here and as supported by International Union of Pure and Applied Chemistry reports dating from 1988 and 2021.

Francium and radium make up the s-block elements of the 7th period.

Francium (Fr, atomic number 87) is a highly radioactive metal that decays into astatine, radium, or radon. It is one of the two least electronegative elements; the other is caesium. As an alkali metal, it has one valence electron. Francium was discovered by Marguerite Perey in France (from which the element takes its name) in 1939. It was the last element discovered in nature, rather than by synthesis. Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where the isotope francium-223 continually forms and decays. As little as 20–30 g (one ounce) exists at any given time throughout Earth's crust; the other isotopes are entirely synthetic. The largest amount produced in the laboratory was a cluster of more than 300,000 atoms.