Table 4.1.Revised 2011 by D.E.Groom(LBNL),and E.Bergren.Atomic weights of stable elements are adapted from the Commission on Isotopic Abundances and Atomic Weights,“Atomic Weights of the Elements 2007,”http://www.chem.qmul.ac.uk/iupac/AtWt/.The atomic number(top left)is the number of protons in the nucleus.The atomic mass(bottom)of a stable elements is weighted by isotopic abundances in the Earth’s surface.If the element has no stable isotope,the atomic mass(in parentheses)of the most stable isotope currently known is given.In this case the mass is from http://www.nndc.bnl.gov/amdc/masstables/Ame2003/mass.mas03 and the longest-lived isotope is from www.nndc.bnl.gov/ensdf/zajform.jsp.The exceptions are Th,Pa,and U,which do have characteristic terrestrial compositions.Atomic masses are relative to the mass of 12C,defined to be exactly 12 unified atomic mass units(u)(approx.g/mole).Relative isotopic abundances often vary considerably,both in natural and commercial samples;this is reflected in the number of significant figures given for the atomic mass.IUPAC does not accept the claims for elements 113,115,117,and 118 as conclusive at this time. Table 4.1. Revised 2011 by DEGroom (LBNL), and E. Berggren. Atomic weights of stable elements are adapted from the Commission on Isotopic Abundances and Atomic Weights, “Atomic Weights of the Elements 2007,” http: // www .chem.qmul.ac.uk / iupac / AtWt /. The atomic number (top left) is the number of protons in the nucleus. The atomic mass (bottom) of a stable elements is weighted by isotopic abundances in the Earth’s surface. If the element has no stable isotope, the atomic mass (in parentheses) of the most stable isotope currently known is given. In this case the mass is from http://www.nndc.bnl.gov/amdc/masstables/Ame2003/ mass.mas03 and the longest-lived isotope is from www.nndc.bnl.gov/ensdf/zajform.jsp.The exceptions are Th, Pa, and U, which do have characteristic terrestrial compositions. Atomic masses are relative to the mass of 12C, defined to be exactly 12 unified atomic mass units (u) (approx.g / mole). Relative isotopic abundances often vary considerably, both in natural and commercial samples; this is reflec ted in the number of significant figures given for the atomic mass. IUPAC does not accept the claims for elements 113, 115, 117, and 118 as conclusive at this time.