element

Substance that cannot be split chemically into simpler substances. The atoms of a particular element all have the same number of protons in their nuclei (their proton or atomic number). Elements are classified in the periodic table of the elements. Of the known elements, 92 are known to occur naturally on Earth (those with atomic numbers 1-92). Those elements with atomic numbers above 96 do not occur in nature and must be synthesized in particle accelerators. Of the elements, 81 are stable; all the others, which include atomic numbers 43, 61, and from 84 up, are radioactive.

Elements are classified as metals, nonmetals, or metalloids (weakly metallic elements) depending on a combination of their physical and chemical properties; about 75% are metallic. Some elements occur abundantly (oxygen, aluminium); others occur moderately or rarely (chromium, neon); some, in particular the radioactive ones, are found in minute (neptunium, plutonium) or very minute (astatine, technetium) amounts. Symbols (devised by Swedish chemist Jöns Berzelius) are used to denote the elements; the symbol is usually the first letter or letters of the English or Latin name (for example, C for carbon, Ca for calcium, Fe for iron, from the Latin ferrum). The symbol represents one atom of the element. Two or more elements bonded together form a compound from which they cannot be separated by physical means. Compounds are held together by ionic or covalent bonds. The number of atoms of an element that combine to form a molecule is it atomicity. A molecule of oxygen (O₂) has atomicity 2; sulphur (S₈) has atomicity 8.

According to current theories, hydrogen and helium were produced in the Big Bang at the beginning of the universe. Of the other elements, those up to atomic number 26 (iron) are made by nuclear fusion within the stars. The heavier elements, such as lead and uranium, are produced when an old star explodes; as its centre collapses, the gravitational energy squashes nuclei together to make new elements.

New elements Elements 110 and 111 were discovered in 1994. Element 110 was detected for a millisecond at the GSI heavy-ion cyclotron in Darmstadt, Germany, while lead atoms were bombarded with nickel atoms. It has an atomic mass of 269. Element 111 was later detected at GSI, when bismuth-209 was bombarded with nickel. It has an atomic mass of 272. Element 112 was discovered there in 1996. It has an atomic mass of 277. After firing 5 billion billion zinc ions at a speed of 30,000 kps/18,640 mps at lead, the German scientists created a single atom of element 112 that survived for a third of a millisecond. Russian scientists at the Institute for Nuclear Research at Dubna created element 114 in 1999. The element, produced by colliding isotopes calcium-48 and plutonium-44, lasted for 20 seconds. Element 114, confirms the existence of an ‘island’ of stable heavy elements. US physicists successfully created element 118 by bombarding lead with krypton. It then decayed into element 116, another new element. However, in 2001 the claim regarding the discovery of element 118 was refuted; the new elements existed only for milliseconds.

In 2004, the creation of two new elements was reported by a team of US and Russian scientists: elements 113 and 115. Four atoms of element 115 were created by bombarding an Americanium target with calcium atoms, and these decayed after 90 milliseconds to form element 113, which lasted for 1.2 seconds. Elements 113 and 115 were called ununtrium and ununpentium, pending their confirmation by the international scientific community.