Explain what the relative atomic mass of an element is.

Atomic mass or relative mass (or can also be called as – atomic weight) – the value of the mass of an atom, expressed in atomic mass units. It is defined as the ratio of the mass of an atom of a given element to 1⁄12 of the mass of a neutral atom of the carbon isotope 12C. It follows from the definition that the relative atomic mass is a dimensionless quantity. Dimensionless quantity is a physical quantity, in the dimension of which all factors corresponding to the basic physical quantities of a given system of physical quantities are included in a degree equal to zero.