The periodic table of the chemical elements (also periodic table of the elements or just the periodic table) is a tabular display of the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior. Modern quantum mechanical theories of atomic structure explain group trends by proposing that elements within the same group have the same electron configurations in their valence shell, which is the most important factor in accounting for their similar properties. Elements in the same group also show patterns in their atomic radius, ionization energy, andelectronegativity. From top to bottom in a group, the atomic radii of the elements increase. Since there are more filled energy levels, valence electrons are found farther from the nucleus. From the top, each successive element has a lower ionization energy because it is easier to remove an electron since the atoms are less tightly bound. Similarly, a group will also see a top to bottom decrease in electronegativity due to an increasing distance between valence electrons and the nucleus.

Californium is a synthetic radioactive metallic chemical element in the actinide series with the symbol Cf and atomic number 98. The element was first produced in 1950 by bombarding curium with alpha particles (helium ions) at theUniversity of California, Berkeley. It was the sixth transuranium element to be synthesized and is one of highest atomic mass elements to be produced in weighable amounts. The element was named for California and the University of California.Californium is one of the few transuranium elements that have practical applications. Most of these applications exploit the property of certain isotopes of californium to emit neutrons. For example, californium can be used to help start-up nuclear reactors, and is employed as a source of neutrons when studying materials with neutron diffraction and neutron spectroscopy. Element 118 was synthesized by bombarding californium-249 atoms with calcium-48 ions.Californium was first synthesized at the University of California, Berkeley by the physics researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg on or about February 9, 1950. It was the sixth transuranium element to be discovered, and this team announced its discovery on March 17, 1950. To produce californium, a microgram-sized target of curium-242 was bombarded with 35 MeV alpha particles in the 60-inch-diameter (1,500 mm) cyclotron at Berkeley, California, which produced nuclei of californium-245 (half-life 44 minutes), plus one free neutron.24296Cm + 42He → 24598Cf + 10n Only about 5,000 atoms of californium were produced in this experiment. The discoverers named the new element for California and also the University of California. This name was a break from the convention that had been used for the elements 95 to 97, which drew their inspiration from how the elements directly above them in the periodic table were named.Europium, in the sixth period directly above element 95, was named for the continent it was discovered on, so element 95 was named americium. Element 96 was named for Marie Curie andPierre Curie as an analog to the naming of gadolinium, which was named for the scientist andengineer Johan Gadolin. Terbium was named for the city it was discovered in, so element 97 was named berkelium. However, the element directly above element 98 in the periodic table, dysprosium, has a name that simply means "hard to get at" so the researchers decided to set-aside the informal naming convention. They added that "the best we can do is to point out [that] ... searchers a century ago found it difficult to get to California."Weighable quantities of californium were first produced by long-duration irradiation of plutonium targets at the Materials Testing Reactor at theIdaho National Laboratory. The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958.[27] Californium-249 to -252 were isolated that same year from a sample of plutonium-239that had been irradiated with neutrons in a nuclear reactor for five years. The High Flux Isotope Reactor (HFIR) at the Savannah River Site in South Carolina, started producing small batches of californium in the 1960s. The Atomic Energy Commission began selling, leasing, or lending small amounts of californium-252 to industrial and academic customers in the early 1970s for ten dollars per microgram; an average of 150 mg of californium-252 were shipped each year from 1970 to 1990. Milligram-quantities of californium can only be made in specialized high-flux reactors; there are only two reactors operating that can efficiently produce it: the High Flux Isotope Reactor in the United States and the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. By 1995, the HFIR nominally produced 502 grams of californium annually. The difficulty of obtaining bulk quantities of californium led, in 1974, to the misidentification of hexagonal Cf2O2S and face-centered cubic CfS as two forms of californium metal. The crystal structure needed to be determined by using microgram amounts of the element