Guide to the Periodic Table: Finding New Elements
How is the Periodic Table of Elements Organized?
The Periodic Table of the Elements is a way of organizing all the known elements in terms of their chemical and physical properties. The atomic number (the number of protons) increases along a row from left to right. Elements along a row have the same number of principal electron shells or energy levels. Elements in the same column, (for the first two and last six columns) have the same number of outer shell or valence electrons. Transitional metals (elements in the yellow colored boxes) do not follow this rule.
This version of the Periodic Table also shows the phases -solid, liquid or gas- at room temperature of the naturally occurring elements on Earth.
What is an Isotope?
An element is determined by the number of protons (atomic number) contained in its nucleus. Isotopes of an element have different numbers of neutrons. The mass number is the total number of protons and neutrons present in the nucleus. For example, the element hydrogen has 3 isotopes: 1H has just one proton and no neutron, 2H has 1 proton and 1 neutron (this isotope of hydrogen is often called deuterium–D), 3H has 1 proton and 2 neutrons (and is often called tritium–T). The three isotopes of hydrogen have very similar properties—for example, they all combine with oxygen to make water: H2O, D2O and T2O.
The atomic mass–u (or atomic weight) is the mass of an atom in atomic mass units. (One amu = 1.661 x 10 –24 g, or exactly 1/12th of the mass of one 12C atom). The mass given for each element in the table is the weighted average of all of the element’s isotopic masses. The exceptions are the synthetic elements, where the atomic mass of the longest-lived isotope is listed.
What is Radioactive Decay?
Radioactive decay refers to processes where the nucleus of an isotope emits particles such as electrons and photons (gamma rays). When the radioactive nucleus emits a charged particle, the atomic number of the nucleus changes and becomes another element. Two of the radioactive decay processes are described below.
What is Alpha Decay?
An alpha particle is a 4He nucleus; it has 2 protons and 2 neutrons. Alpha decay refers to a process by which an atomic nucleus emits an alpha particle. The new, "daughter", nucleus now has 2 fewer protons and 2 fewer neutrons than the "parent" nucleus. The daughter nucleus is a different element from the parent. Heavy elements typically "decay" through alpha particle emission.
Element 281112 has 112 protons and 169 neutrons. After alpha decay, the daughter is element 277110 (it has 110 =112-2 protons and 167=169-2 neutrons).
What is Beta Decay?
Electrons are sometimes called beta particles. In beta decay, a neutron is converted to a proton and an electron; the nucleus emits the electron. The daughter nucleus now has 1 more proton, but 1 neutron less than the parent does. For example, 14C (carbon 14) has 6 protons and 8 neutrons. After beta decay, the daughter is 14N with 7 protons and 7 neutrons.
How Are Massive Elements Made?
Of the 94 elements that exist in nature, the most massive element found in sizeable quantities is uranium. Technetium and Plutonium are found in nature, but in very minute quantities. Stars, in their cores or in supernova explosions, make all the naturally existing and synthetic elements (though hydrogen and helium were made in the Big Bang). However, many decay very soon after they are made. The phrase, synthetic elements, refers to elements that have been made in a laboratory or nuclear reactor..
For more than 60 years, physicists have used nuclear reactors and particle accelerators to make transuranic elements (elements with atomic numbers greater than 92). All these elements eventually decay.
Although some transuranic isotopes have mean half-lives of thousands of years (244Pu has a half-life of 83,000,000 years), most are very short-lived. Element 277112 is so unstable it lasts for only 0.24 milliseconds (0.000240 of a second).
Theorists have predicted, however, that this trend toward instability would be reversed as additional protons and neutrons fill out nuclear shells. Elements with atomic numbers between 110 and 120 are predicted to be on an island of stability.
The Discovery of Element 114
A collaboration of scientists from the Joint Institute for Nuclear Research in Dubna, Russia and from the Lawrence Livermore National Laboratory in Livermore, California announced in January 1999 that they had made element 114. For several weeks a 244Pu target was bombarded with about 5 x1018 atoms of 48Ca, a rare calcium isotope. After analyzing their data, the scientists detected a three-alpha decay chain that appears to be the unique signature of a decay chain starting with element 289114. Element 114 appeared to last for 30 seconds before forming an isotope of element 112 through alpha decay. Because only one signature was observed, this exciting discovery awaits confirmation. If confirmed, element 114’s longevity could support theorists’ prediction of an island of stability.
The Discovery of Elements 116 and 118
In May, 1999 scientists at the Lawrence Berkeley National Laboratory’s 88-inch Cyclotron in Berkeley, California announced their discovery of element 118 and its immediate decay product, element 116. During eleven days, a 208Pb target was bombarded by a beam of 86Kr
In less than a millisecond after its creation, the element 118 nucleus decayed through a chain of 6 alpha particles to an isotope of element 106 (seaborgium). The 116289 isotope of element 116 is a product of the decay chain. Three such alpha-decay chains were observed indicating production of three atoms of element 118. The decay times and energies measured for these new isotopes of elements 118, 116, 114, 112, 110, 108, and 106 provides a hint of the existence of the predicted island of stability–a group of elements that live much longer then lower mass elements.
Diagram depicting the formation of Element 118