Man-Made Elements

FIELDS OF STUDY: Nuclear Chemistry; Inorganic Chemistry

Abstract

The occurrence and properties of man-made elements are discussed. Man-made elements are the transuranium elements in the actinide series and beyond. Uranium is the heaviest naturally occurring actinide element and is commonly used as a starting material in nuclear synthesis reactions. All man-made elements are radioactive.

The Nature of Man-Made Elements

Only the first ninety-eight elements of the periodic table, hydrogen through californium, are known to occur naturally, yet the periodic table lists well over one hundred elements. All of the transuranium elements, which are all elements with an atomic number greater than that of uranium (92), are radioactive, and all were first discovered by being artificially created in a laboratory. Each isotope of these elements has a specific half-life, some of which are very short, measured in mere fractions of a second. Others, such as plutonium, have half-lives of several thousand years. All transuranium elements were originally thought to be entirely synthetic, or man-made, but atomic numbers 93 (neptunium) through 98 (californium) were later discovered to occur in nature in trace amounts.

One consistent feature of man-made elements is that they are very unstable and have short half-lives. The longest-lived known isotope of a purely synthetic element is einsteinium-252, which has a half-life of 471.7 days. Thus, even if some of these elements existed naturally in the distant past, they would have long since decayed into lower elements. Radioactive isotopes, or radioisotopes, decay in a series of nuclear-fission events in which their nuclei lose mass by spontaneously emitting particles in the form of ionizing radiation, ultimately transforming the isotopes into stable elements with less massive nuclei.

Accelerating charged particles to high kinetic energies can initiate the opposite type of reaction, nuclear fusion, in which highly energetic atomic nuclei are made to collide and fuse with a uranium or transuranium nucleus, adding new protons and neutrons to produce a more massive nucleus. The addition of a carbon-12 nucleus to a curium-246 nucleus, for example, produces the man-made isotope nobelium-254. A similar method is neutron capture, in which the original nucleus is collided with individual neutrons rather than whole nuclei. Such reactions may require a number of years of continual bombardment to produce barely enough material to examine, and some of these elements have been identified from electronic traces produced by no more than a few individual atoms.

In November 2016, the International Union of Pure and Applied Chemistry (IUPAC) approved official names for four man-made elements—nihonium (atomic number 113), moscovium (115), tennessine (117), and oganesson (118)—that were first synthesized in 2003 or later. Previously these elements were listed in the periodic table under placeholder names. With the official inclusion of these four elements, all remaining spots in the current periodic table have been filled. Any further discoveries of new man-made elements will necessitate the addition of an eighth row to the table.

Electronic Structures

The man-made elements that have been studied most extensively are those in the actinide series, which all have valence electrons in their f orbitals and appear to exhibit the properties expected of elements with such electron configurations. The f orbitals can hold up to fourteen electrons, and a number of oxidation states are possible for these elements, although the purely synthetic actinides—einsteinium, fermium, mendelevium, nobelium, and lawrencium—are less variable in their oxidation states than the others. Atoms appear to be most stable when the outermost electron shell on the atom corresponds to that of the nearest noble gas. Thus, elements toward the left-hand side of the periodic table are prone to give up electrons, while elements toward the right-hand side of the periodic table are prone to accept extra electrons into their valence shells. An intermediate stability level is attained when the outermost electron shell is half filled. The man-made actinides most commonly take on a +3 oxidation state—that is, they lose three electrons to become a positively charged ion, or cation—although nobelium is most stable in a +2 oxidation state, and einsteinium, fermium, and mendelevium are occasionally stable in the +2 state as well.

Applications

Most of the elements heavier than plutonium do not exist in sufficient quantities to have any practical applications. Uranium, generally the fuel of choice for nuclear reactors, is also the starting material for the production of plutonium—perhaps the most virulently poisonous material known to exist, as well as being dangerously radioactive and the fuel of choice for nuclear warheads. While miniscule amounts of plutonium are found in nature, typically as decay products in concentrated uranium deposits, most of the plutonium that exists was created in nuclear reactors by bombarding uranium-238 with neutrons or deuterons (the nuclei of deuterium, or heavy hydrogen). Plutonium is a high-output fuel for nuclear reactors, but it has a half-life of nearly twenty-five thousand years, making storage of depleted plutonium fuel rods a serious safety problem in terms of both containing it and preventing it from being repurposed for destructive uses.

Principal Terms

• half-life: the length of time required for one-half of a given amount of material to decompose or be consumed through a continuous decay process.
• plutonium: atomic number 94, an extremely toxic and dangerously radioactive element of the actinide group that has several known isotopes.
• radioactivity: the emission of subatomic particles due to the spontaneous decay of an unstable atomic nucleus, the process ending with the formation of a stable atomic nucleus of lower mass.
• synthetic: produced by artificial means or manipulation rather than by naturally occurring processes.
• transuranium elements: the elements in the periodic table that have an atomic number greater than that of uranium (92), all of which are unstable and radioactive.

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