element

element, in chemistry,a substance composed of atoms all having the same number of protons in their nuclei (atomic number). The atomic number defines the element and establishes its place in the periodic table. Each element is assigned a symbol of one to three letters. The total number of protons and neutrons in the nucleus of an atom is called the mass number. Atoms with the same atomic number but different mass number are different isotopes of the element. All elements have isotopes; over 1,000 isotopes of the elements are known. Although 110 elements are known, only 94 elements occur naturally on earth; the others are produced artificially. The chemical properties of an element are due to the distribution of electrons around the nucleus, particularly the outer, or valence, electrons (the ones involved in chemical reactions). Chemical reaction does not affect the nucleus and thus does not change the atomic number. For this reason an element is often defined as a substance that cannot be decomposed into simpler substances by chemical means. A substance whose smallest unit consists of molecules of the atoms of two or more elements chemically bound together is called a compound.