Element See also: Atomic Weight; Compound; Molecule In chemistry, substance composed of Atoms all having the same number of Protons in their nuclei. This number, called 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(see Chemical Elements). The total number of protons and Neutrons in the nucleus of an atom is called the Mass Number. Although all atoms of an element have the same number of protons in their nuclei, they may not all have the same number of neutrons. Atoms of an element with the same mass number make up an Isotope of the element. All elements have isotopes; over 1,000 isotopes of the elements are known. Although 109 elements are known, only 92 elements occur naturally on earth; the others are produced artificially (see Synthetic Elements; Transuranium Elements ). 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. See Chemical Elements for an alphabetical table of the elements.