Elements

• Chemical element is a chemical substance that cannot be broken down into other substances by chemical reactions
• Atom


  The basic particle that constitutes a chemical element
• Atomic number


  The number of protons in the nuclei of an element's atoms
• Oxygen has an atomic number of 8, meaning that each oxygen atom has 8 protons in its nucleus
• Molecule


  Two or more atoms of the same element can combine to form molecules
• Chemical compound


  Contains atoms of different elements
• Chemical mixture


  Contains atoms of different elements
• Atoms can be transformed into different elements in nuclear reactions, which change an atom's atomic number
• Almost all of the baryonic matter of the universe is composed of chemical elements
• When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds
• Only a few elements, such as silver and gold, are found uncombined as relatively pure native element minerals
• Nearly all other naturally occurring elements occur in the Earth as compounds or mixtures
• Air is primarily a mixture of molecular nitrogen and oxygen, though it does contain compounds including carbon dioxide and water, as well as atomic argon, a noble gas which is chemically inert and therefore does not undergo chemical reactions
• Dmitri Mendeleev published the first recognizable periodic table


  1869
• Periodic table


  Organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties
• By November 2016, the International Union of Pure and Applied Chemistry had recognized a total of 118 elements
• The first 94 elements occur naturally on Earth, and the remaining 24 are synthetic elements produced in nuclear reactions
• Save for unstable radioactive elements (radionuclides) which decay quickly, nearly all of the elements are available industrially in varying amounts
• The discovery and synthesis of further new elements is an ongoing area of scientific study
• The lightest chemical elements are hydrogen and helium, both created by Big Bang nucleosynthesis during the first 20 minutes of the universe
• Almost all other elements found in nature were made by various natural methods of nucleosynthesis
• Of the 94 naturally occurring elements, those with atomic numbers 1 through 82 each have at least one stable isotope (except for technetium, element 43 and promethium, element 61, which have no stable isotopes)
• Elements with atomic numbers 83 through 94 are unstable to the point that radioactive decay of all isotopes can be detected
• The very heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized
• Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, although trace amounts of technetium have since been found in nature
• Radioactive elements


  • List of nuclides, sorted by length of half-life for those that are unstable
• Periodic table


  Groups together elements with similar chemical properties (and usually also similar electronic structures)
• Atomic number


  The number of protons in each atom, and defines the element
• Carbon atoms


  • 6 protons in their atomic nucleus
• Isotopes


  Atoms of the same element (same number of protons) but with different numbers of neutrons
• Carbon has three main isotopes: carbon-12, carbon-13, and carbon-14
• Most (66 of 94) naturally occurring elements have more than one stable isotope
• Except for isotopes of hydrogen, the isotopes of a given element are chemically nearly indistinguishable
• All elements have some isotopes that are radioactive (radioisotopes), although not all occur naturally
• Certain elements have no stable isotopes and are composed only of radioactive isotopes
• 26 elements have only one single stable isotope
• The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element
• The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50)
• Mass number


  The number of nucleons (protons and neutrons) in the atomic nucleus
• Atomic mass


  The mass of a single atom of a particular isotope, typically expressed in daltons or universal atomic mass units