The relative atomic mass, AT, is the average mass of the atoms of the element as it occurs on Earth. It is thus an average over all the isotopes of the element. The scale is fixed by giving the carbon isotope ll C a relative atomic mass of 12.0. By convention, the Solar System abundance is normalized to 1012 atoms of hydrogen, whereas the CI chondrite abundance is normalized to 106 atoms of silicon. To directly compare chondrite abundance to Solar System abundance (by number), you would multiply chondrite abundance by 35.8.