relative atomic mass

Relative atomic mass, also known as atomic weight or average atomic mass, is the mean mass of the atoms of an element weighted to the relative abundance of its naturally occurring isotopes and measured relative to some standard. Since 1961 this standard has been provided by the carbon isotope 12C whose atomic mass is defined to be exactly 12. On this scale relative atomic masses for the naturally occurring elements range from 1.008 (hydrogen) to 238.03 (uranium).