16 S&TR January/February 2002 When they synthesized LEMENTS that do not exist in E nature—that have been created in a elements 114 and 116, laboratory—are unstable. After hours or days of one element bombarding Russian and Livermore another with enough energy for both to fuse, the resulting new element typically scientists confirmed is born and begins to decay instantly. Neptunium and plutonium decades-old (elements 93 and 94) were the first elements created in a laboratory, at the predictions of the University of California at Berkeley in 1940. Scientists have since fabricated existence of superheavy many more elements, each one heavier and with a shorter half-life than the elements with one before it. In the 1960s, a few physicists comparatively long predicted that some elements around element 114 would survive longer than lifetimes. any of their synthesized predecessors. Early estimates for the half-lives of these more stable elements were as high as billions of years. Later computer modeling reduced the anticipated half- lives to seconds or minutes before the element began to decay. Half-lives of seconds or minutes may seem brief. But consider that various atoms of element 110 created in the laboratory have had half-lives ranging from 100 microseconds to 1.1 milliseconds. The only atom of element 112 that had been created before 1998 had a lifetime of 480 microseconds. As described further in the box on p. 21, the long- lived nuclei of elements around element 114 would comprise an “island of stability” in a “sea” of highly unstable elements. When a collaboration of Russian and Livermore scientists at the Joint Institute for Nuclear Research in Lawrence Livermore National Laboratory S&TR January/February 2002 Elements 114 and 116 17 Map of the voyage to the island 114 of stability. Island of stability of superheavy spherical nuclei Shoal of 100 deformed Proton number nuclei Thalium Uranium 90 Plutonium Lead Mountains Continent 126 142 146 162 184 Neutron number Dubna, Russia, created element 114 different mass, thereby making it a representing Livermore and Yuri in 1998, the first atom survived for different isotope of element 114. The Oganessian, scientific director of 30 seconds before it began to decay, team has also created several the Flerov Laboratory of Nuclear a spontaneous process that leads to previously undiscovered isotopes of Reactions at the Joint Institute, the creation of another element with elements 112, 110, and 108 to which leading the Russians. In the early a lower number on the periodic table. element 114 decayed. More recently, 1990s, the U.S.–Russian team (See the box on pp.18–19for more the team added element 116 to the discovered two isotopes of element 106, information on stability and instability.) periodic table with the creation of one isotope of 108, and one of 110 at A total of 34 minutes elapsed before three atoms of the element in a series the Dubna institute. the final decay product fissioned, of experiments. “In 1990, when Ron Lougheed, splitting in two the surviving nucleus. Nuclear chemist Ken Moody who has since retired, and I went to These lifetimes may seem brief, but leads the Livermore portion of the Dubna, we were the first U.S. scientists they are millions of times longer than international collaboration. “In 1998, to perform experiments at that those of other recently synthesized we proved that there really was an institute,” adds Moody. “Remember heavy elements. island of stability,” he said. “We what was happening then. The Berlin Since that groundbreaking effort in proved that years of nuclear theory Wall had just fallen, and Eastern 1998, the team has created another actually worked.” Europe was in turmoil. The early days atom of element 114. This one has a The collaboration began in 1989. of the collaboration were definitely different number of neutrons and thus a with heavy element chemist Ken Hulet interesting.” Lawrence Livermore National Laboratory 18 Elements 114 and 116 S&TR January/February 2002 Photo of Forty Days and Nights Russian– Noah’s flood could have come Livermore and gone in the time it took the team. collaboration to create the first atom of element 114. For 40 days of virtually continuous operation, calcium ions bombarded a spinning target of plutonium in Dubna’s U400 cyclotron. While the first atom of element 114 was actually created on November 22, 1998, Russian researchers discovered it in data analysis and communicated the news to Livermore on December 25, 1998— quite the Christmas present. A Primer on Stability and Instability Why should element 114 be so much more stable and long-lived filled. Lead-208 has 82 protons and 126 neutrons, both of which are than so many of its synthesized predecessors? The answer lies in magic numbers. Lead-208 is thus doubly magic and seems to be basic chemistry. virtually eternal. The nucleus of an atom is surrounded by one or more orbital A long-lived, stable element such as lead does not decay. shells of electrons. The electron configurations of atoms of the many However, all elements with an atomic number greater than 83 elements vary periodically with their atomic number, hence “the (bismuth) exhibit radioactive decay. Decay may happen in several periodic table of the elements.” ways. For heavy elements, an unstable or radioactive isotope usually Elements with unfilled shells seek out electrons in other elements decays by emitting helium nuclei (alpha particles) or electrons (beta to fill them. These include carbon, oxygen, and all of the “reactive” particles), leaving a daughter nucleus of an element with a different elements that want to react with other elements. This is the basis of number of protons. This process typically continues until a stable covalent bonding. The noble gases (on the far right column of the nucleus is reached. Plutonium, for example, decays ultimately to lead. periodic table) have a completely filled outer electron shell and hence The heavy elements that have been created in the laboratory are are highly stable. They are termed noble because they are “aloof,” so unstable that they decay almost immediately and have extremely with no desire to react with other elements. 1 2 Protons and neutrons are in analogous H He Hydrogen Periodic table Helium shells within the nucleus. The proton shells of 3 4 5 6 7 8 9 10 Li Be B C N O F Ne helium, oxygen, calcium, nickel, tin, and lead Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon 11 12 13 14 15 16 17 18 are completely filled and arranged such that Na Mg Al Si P S Cl Ar the nucleus has achieved extra stability. The Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 atomic numbers of these elements—2, 8, 20, K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton 28, 50, and 82—are known as “magic numbers.” 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe These same numbers plus 126 are magic Rubidium Strontium Yitrium Zircnium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon 55 56 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 numbers for neutrons. Notice that the magic See Cs Ba Lantha- Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn numbers are all even. No truly stable element Cesium Barium nides Hafnium Tantaium Tungsten Rhenium Osmium Iridium Platinum Gold Mercury Thallium Lead Bismuth Polonium Astatine Radon 105 106 107 108 109 110 111 112 87 88 See 104 114 116 heavier than nitrogen has an odd number of Fr Ra Acti- Rf Ha Sg Bh Hs Mt both protons and neutrons. Elements with even Francium Radium nides Rutherfordiumm Hahnium Seaborgium Bohrium Hassium Meitnerium numbers of protons and neutrons make up 57–71 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Lanthanides Ln* La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu about 90 percent of Earth’s crust. Lanthanides Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium 89–103 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 A nucleus is “doubly magic” when the Actinides An* Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr shells of both the protons and neutrons are Actinides Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Lawrence Livermore National Laboratory S&TR January/February 2002 Elements 114 and 116 19 The box on p. 22 shows the “recipe” collide with enough energy to fission. In alpha decay, an isotope loses for the early Dubna experiments that overcome their mutual electrostatic an alpha particle, which is two protons created isotopes of element 114. repulsion, the compound nucleus has and two neutrons (or a helium nucleus). Plutonium, with an atomic number, or excess energy. A few neutrons For example, an atom of element 114 Z, of 94, and calcium, Z = 20, add up evaporate to de-excite the nucleus and with 175 neutrons (described as isotope to the necessary Z = 114.