Synthetic Elements
Synthetic elements are chemical species that do not occur naturally on Earth and must be produced artificially through nuclear reactions. From technetium, the first element created in a laboratory, to the superheavy oganesson at the bottom of the periodic table, these elements have expanded our understanding of nuclear physics, challenged the limits of the periodic table, and opened the door to the hypothetical "island of stability."
Technetium: The First Synthetic Element
In 1937, Carlo Perrier and Emilio Segrè identified technetium (Tc, atomic number 43) in a sample of molybdenum that had been bombarded with deuterons in Ernest Lawrence's cyclotron at Berkeley. Technetium was the first element produced artificially, filling a long-standing gap in the periodic table. All of its isotopes are radioactive, with the longest-lived — Tc-98 — having a half-life of about 4.2 million years. Despite its instability, technetium is produced in kilogram quantities in nuclear reactors and is widely used in medical imaging: Tc-99m is the most common radioisotope in diagnostic medicine.
Transuranic Elements
Elements beyond uranium (Z > 92) are called transuranic elements. Neptunium (Np, 93) and plutonium (Pu, 94) were first synthesized in the early 1940s by Glenn Seaborg and colleagues by bombarding uranium with neutrons and deuterons. Plutonium became critically important for nuclear weapons and power generation.
The elements from americium (95) through californium (98) were produced during the 1940s and 1950s, primarily through successive neutron capture in nuclear reactors followed by beta decay. Einsteinium (99) and fermium (100) were first detected in the debris of the 1952 Ivy Mike thermonuclear test — they formed when uranium-238 captured multiple neutrons in the intense neutron flux of the explosion.
Synthesis Methods
Particle accelerators are the primary tool for creating superheavy elements (Z > 100). A beam of lighter ions is accelerated to high energy and directed at a heavy target nucleus. If the two nuclei fuse, a compound nucleus forms. The key challenge is that the compound nucleus is produced in a highly excited state and usually fissions immediately. Only a tiny fraction survive by emitting neutrons and gamma rays to reach a bound state.
Two main strategies exist:
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Hot fusion — Lighter projectiles (such as calcium-48) are fired at actinide targets (such as californium-249). The compound nucleus is formed at high excitation energy and must evaporate several neutrons. This approach, pioneered at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, produced elements 114 through 118.
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Cold fusion — Heavier projectiles (such as iron-58 or zinc-70) are used on lead-208 or bismuth-209 targets. The compound nucleus forms with lower excitation energy, needing to shed only one or two neutrons. This method, developed at the GSI Helmholtz Centre in Darmstadt, Germany, was used to synthesize elements 107 through 112.
Neutron capture, which occurs in nuclear reactors and stellar explosions, can build up heavier isotopes step by step. However, this process becomes inefficient beyond fermium because the resulting isotopes undergo spontaneous fission faster than they can capture additional neutrons.
The Island of Stability
Nuclear shell theory predicts that certain "magic numbers" of protons and neutrons create especially stable configurations, analogous to the closed electron shells of noble gases. The predicted doubly magic nucleus near Z = 114 and N = 184 is expected to sit at the center of an "island of stability" where superheavy isotopes might have half-lives of minutes, years, or even millions of years — dramatically longer than the millisecond lifetimes of nearby nuclides.
Experiments at JINR have provided tantalizing hints: flerovium-289 (Z = 114) has a half-life of about 2 seconds, much longer than neighboring elements. However, the predicted most-stable isotopes with 184 neutrons have not yet been synthesized, because current techniques cannot produce nuclei with that neutron count.
Half-Lives and Detection
Most superheavy elements exist for fractions of a second. Oganesson-294 (Z = 118) has a half-life of about 0.7 milliseconds. Detection relies on observing the chain of alpha decays that follow synthesis: each decay produces a characteristic energy signature, and the chain terminates at a known, longer-lived isotope, confirming the identity of the original superheavy atom.
Experiments sometimes produce only one or two atoms over months of bombardment. The synthesis of element 117 (tennessine) in 2010, for example, yielded six atoms after several months of continuous beam time.
Naming Conventions
The International Union of Pure and Applied Chemistry (IUPAC) governs the naming of new elements. Until officially named, elements receive systematic placeholder names based on their atomic number using Latin and Greek roots: ununtrium (113), flerovium (114), moscovium (115), livermorium (116), tennessine (117), and oganesson (118) were the last batch to receive permanent names in 2016.
Naming often honors scientists (Seaborg, Flerov, Oganesson), laboratories (Lawrence Livermore, JINR), or geographic locations (Moscow, Tennessee). The process typically takes several years after discovery is confirmed and can involve considerable international negotiation.
Looking Forward
Research teams worldwide are now attempting to synthesize elements 119 and 120, which would begin a new row of the periodic table and enter the predicted region of superactinide chemistry. Success will test whether our understanding of nuclear structure and relativistic quantum mechanics holds at the extreme frontier of the elements.