The atomic number (top left) is the number of protons in the nucleus. The atomic mass (bottom) is weighted by mean isotopic abundances in the earth s surface. Atomic masses are relative to the mass of the carbon-12 isotope, defined to be exactly 12 atomic mass units (amu). If sufficiently stable isotopes do not exist for an element, the mass of its longest-lived isotope is indicated in parentheses. For elements 110 112, the mass numbers of the known isotopes are given. Reference: Schroeder (2000) as extracted from The European Physical Journal C3, 73 (1998).