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 behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour 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; however, there are exceptions, such as plutonium.
All elements of period 7 are radioactive. This period contains the actinides, which contains the heaviest naturally occurring element, plutonium; subsequent elements must be synthesized artificially. Whilst the first five of these 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.
Although the rarity of many of these elements means that experimental results are not very extensive, 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 behaviour 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 positive electrical charge from their massive atomic nuclei.
Chemical element Chemical series Electron configuration Occurrence 87 Fr Francium Alkali metal [Rn] 7s1 From decay 88 Ra Radium Alkaline earth metal [Rn] 7s2 From decay 89 Ac Actinium Actinide [Rn] 6d1 7s2 From decay 90 Th Thorium Actinide [Rn] 6d2 7s2 (*) Primordial 91 Pa Protactinium Actinide [Rn] 5f2 6d1 7s2 From decay 92 U Uranium Actinide [Rn] 5f3 6d1 7s2 Primordial 93 Np Neptunium Actinide [Rn] 5f4 6d1 7s2 From decay 94 Pu Plutonium Actinide [Rn] 5f6 7s2 (*) From decay 95 Am Americium Actinide [Rn] 5f7 7s2 (*) Synthetic 96 Cm Curium Actinide [Rn] 5f7 6d1 7s2 Synthetic 97 Bk Berkelium Actinide [Rn] 5f9 7s2 (*) Synthetic 98 Cf Californium Actinide [Rn] 5f10 7s2 (*) Synthetic 99 Es Einsteinium Actinide [Rn] 5f11 7s2 (*) Synthetic 100 Fm Fermium Actinide [Rn] 5f12 7s2 (*) Synthetic 101 Md Mendelevium Actinide [Rn] 5f13 7s2 (*) Synthetic 102 No Nobelium Actinide [Rn] 5f14 7s2 (*) Synthetic 103 Lr Lawrencium Actinide [Rn] 5f14 7s2 7p1 (*) Synthetic 104 Rf Rutherfordium Transition metal [Rn] 5f14 6d2 7s2 Synthetic 105 Db Dubnium Transition metal [Rn] 5f14 6d3 7s2 Synthetic 106 Sg Seaborgium Transition metal [Rn] 5f14 6d4 7s2 Synthetic 107 Bh Bohrium Transition metal [Rn] 5f14 6d5 7s2 Synthetic 108 Hs Hassium Transition metal [Rn] 5f14 6d6 7s2 Synthetic 109 Mt Meitnerium Transition metal (?) [Rn] 5f14 6d7 7s2 (?) Synthetic 110 Ds Darmstadtium Transition metal (?) [Rn] 5f14 6d8 7s2 (?) Synthetic 111 Rg Roentgenium Transition metal (?) [Rn] 5f14 6d9 7s2 (?) Synthetic 112 Cn Copernicium Post-transition metal [Rn] 5f14 6d10 7s2 (?) Synthetic 113 Nh Nihonium Post-transition metal (?) [Rn] 5f14 6d10 7s2 7p1 (?) Synthetic 114 Fl Flerovium Post-transition metal (?) [Rn] 5f14 6d10 7s2 7p2 (?) Synthetic 115 Mc Moscovium Post-transition metal (?) [Rn] 5f14 6d10 7s2 7p3 (?) Synthetic 116 Lv Livermorium Post-transition metal (?) [Rn] 5f14 6d10 7s2 7p4 (?) Synthetic 117 Ts Tennessine Post-transition metal (?) [Rn] 5f14 6d10 7s2 7p5 (?) Synthetic 118 Og Oganesson Noble gas (?) [Rn] 5f14 6d10 7s2 7p6 (?) Synthetic
(*) Exception to the Madelung rule.
Francium and radium
Francium and radium make up the s-block elements of the 7th period.
Francium is a chemical element with symbol Fr and atomic number 87. It was formerly known as eka-caesium and actinium K.[note 1] It is one of the two least electronegative elements, the other being caesium. Francium is a highly radioactive metal that decays into astatine, radium, and radon. 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.[note 2] 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 the 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.
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1601 years and decays into radon gas. Because of such instability, radium is luminescent, glowing a faint blue. Radium, in the form of radium chloride, was discovered by Marie Skłodowska-Curie and Pierre Curie in 1898. They extracted the radium compound from uraninite and published the discovery at the French Academy of Sciences five days later. Radium was isolated in its metallic state by Marie Curie and André-Louis Debierne through the electrolysis of radium chloride in 1910. Since its discovery, it has given names such as radium A and radium C2 to several isotopes of other elements that are decay products of radium-226. In nature, radium is found in uranium ores in trace amounts as small as a seventh of a gram per ton of uraninite. Radium is not necessary for living organisms, and adverse health effects are likely when it is incorporated into biochemical processes because of its radioactivity and chemical reactivity.
The actinide series derives its name from the group 3 element actinium. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; actinium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence.
Of the actinides, thorium and uranium occur naturally in substantial, primordial, quantities. The radioactive decay of uranium produces transient amounts of actinium, protactinium and plutonium, and atoms of neptunium are occasionally produced from transmutation reactions in uranium ores. The other actinides are purely synthetic elements, although the first six actinides after plutonium would have been produced during the Oklo phenomenon (and long since decayed away), and curium almost certainly previously existed in nature as an extinct radionuclide. Nuclear weapons tests have released at least six actinides heavier than plutonium into the environment; analysis of debris from a 1952 hydrogen bomb explosion showed the presence of americium, curium, berkelium, californium, einsteinium and fermium.
All actinides are radioactive and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in nuclear reactors and nuclear weapons. Uranium and thorium also have diverse current or historical uses, and americium is used in the ionization chambers of most modern smoke detectors.
In presentations of the periodic table, the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table, with placeholders or else a selected single element of each series (either lanthanum or lutetium, and either actinium or lawrencium, respectively) shown in a single cell of the main table, between barium and hafnium, and radium and rutherfordium, respectively. This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table (32 columns) shows the lanthanide and actinide series in their proper columns, as parts of the table's sixth and seventh rows (periods).
Transactinide elements (also, transactinides, or super-heavy elements) are the chemical elements with atomic numbers greater than those of the actinides, the heaviest of which is lawrencium (103). All transactinides of period 7 have been discovered, up to oganesson (element 118).
Transactinide elements are also transuranic elements, that is, have an atomic number greater than that of uranium (92), an actinide. The further distinction of having an atomic number greater than the actinides is significant in several ways:
- The transactinide elements all have electrons in the 6d subshell in their ground state (and thus are placed in the d-block).
- Except for the first two transactinides, rutherfordium and dubnium, even the longest-lasting isotopes of transactinide elements have extremely short half-lives, measured in seconds, or smaller units.
- The element naming controversy involved the first five or six transactinide elements. These elements thus used three-letter systematic names for many years after their discovery had been confirmed. (Usually the three-letter symbols are replaced with two-letter symbols relatively shortly after a discovery has been confirmed.)
Transactinides are radioactive and have only been obtained synthetically in laboratories. None of these elements has ever been collected in a macroscopic sample. Transactinide elements are all named after nuclear physicists and chemists or important locations involved in the synthesis of the elements.
Chemistry Nobel Prize winner Glenn T. Seaborg, who first proposed the actinide concept which led to the acceptance of the actinide series, also proposed the existence of a transactinide series ranging from element 104 to 121 and a superactinide series approximately spanning elements 122 to 153. The transactinide seaborgium is named in his honor.
IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, the time needed for the nucleus to form an electronic cloud.
- Actually the least unstable isotope, francium-223
- Some synthetic elements, like technetium, have later been found in nature.
- Luis A. Orozco (2003). "Francium". Chemical and Engineering News.
- The Manhattan Project. An Interactive History. US Department of Energy
- Gray, Theodore (2009). The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog & Leventhal Publishers. p. 240. ISBN 978-1-57912-814-2.
- Actinide element, Encyclopædia Britannica on-line
- Although "actinoid" (rather than "actinide") means "actinium-like" and therefore should exclude actinium, that element it is usually included in the series.
- Connelly, Neil G.; et al. (2005). "Elements". Nomenclature of Inorganic Chemistry. London: Royal Society of Chemistry. p. 52. ISBN 978-0-85404-438-2.
- Greenwood, p. 1250
- Fields, P.; Studier, M.; Diamond, H.; Mech, J.; Inghram, M.; Pyle, G.; Stevens, C.; Fried, S.; Manning, W. (1956). "Transplutonium Elements in Thermonuclear Test Debris". Physical Review. 102 (1): 180. Bibcode:1956PhRv..102..180F. doi:10.1103/PhysRev.102.180.
- IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004) Archived 2006-10-27 at the Wayback Machine (online draft of an updated version of the "Red Book" IR 3–6)
- Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, eds. (2006). The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer. ISBN 978-1-4020-3555-5.