The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope) 

What Is Atomic Mass And Average Atomic Mass?

 The atomic mass of an element is the mass of one atom of that element. They can be measured in a fairly accurate manner with the help of mass spectrometry. Via experimental means, the mass of one atom can be measured relative to the mass of another. Since hydrogen is the first element (its atomic number is 1), the relative atomic mass of hydrogen is 1. The unit of atomic mass is the atomic mass unit, which is defined as the mass equal to 1/12 of the mass of a single carbon-12 atom. It is approximately equal to 1.66*10-24 grams.Average atomic mass takes into account the isotopic abundance (relative to each other found in the Earth). For example, the most abundant isotope of carbon is carbon-12, which has a relative abundance of 98.89%. The atomic masses of carbon-13 and carbon-14 are 13.003 amu and 14.003 amu respectively. The average atomic mass of carbon is 12.011 Gram atomic mass is the mass of one mole of an element. It is determined by taking the atomic weight for an element on the periodic table and expressing it in grams. So, for example, sodium (Na) has an atomic weight of 22.99 u, so it has a gram atomic mass of 22.99 grams. Gram molecular mass is the mass in grams of one mole of a molecular substance. Gram molecular mass is the same as molar mass. The only difference is that gram molecular mass specifies the mass unit to be used. Gram molecular mass may be reported in grams or grams per mole (g/mol).