element

The chemistry term element (chemical element) consists of all material that cannot be (chemically) changed or broken down into other materials. Such an element consists of all atoms with some given number of protons, e.g., all atoms with 10 protons comprise one particular element and those with 11 comprise another. Currently, 118 elements have been discovered or created. The proton count is termed its atomic number (Z). Two atoms may be of the same element even if their number of neutrons differs, in which case they are termed different isotopes of the same element.

The total number of nucleons (protons and neutrons in the atomic nucleus) is termed the mass number (or atomic mass number), so two isotopes of the same element will have the same atomic number but differ in mass numbers. The term atomic weight (or relative atomic mass) is the average mass per atom in daltons (1/12 the mass of carbon with mass number 12, i.e., of carbon-12) in a sample of the element. (Atomic weight is the traditional term but the latter term is more reasonable since it is a measure of mass rather than weight.) The atomic weight reflects the relative abundances of the isotopes in the sample. The term standard atomic weight is used for the known atomic weight of an element as found on Earth, i.e., reflecting such abundances on Earth. Example elements: