CBSE Class 9 Answered

The mass of an atom is known as the atomic mass.

It is difficult to determine the mass of an individual atom. So we use Relative atomic mass. It can be determined by comparing the mass of a particular atom with that of an atom of standard reference.

According to IUPAC adopted one-twelfth of the mass of a carbon-12 isotope as the standard unit to measure relative atomic masses.

The relative atomic mass is of an element is the number of times an element is heavier than an atom of Carbon-12 isotope.

Actually, the atomic mass we use is the relative atomic mass and not the actual mass of any element. Hence, if oxygen has an atomic mass 16 a.m.u it means its one atom is 16 times heavier than a C-12 atom.