Atomic Molecular Mass

It is known that the atomic mass of an element/compound is not the mass of 1 atom or molecule of the said compound as they occur as a mixture of isotopes. As the atomic and molecular masses are expressed on a relative scale based on a mass of ¹²C atom, all atomic and molecular masses are actually the weighted average of the mass of these isotopes. Therefore, atomic mass of an element can be defined as the average relative mass of the atom of an element as compared to the mass of the atom of carbon (¹²C ) taken as 12.

Relative atomic mass and atomic weight:
The relative atomic mass is represented as A_r of the element is ratio of the mass of an atom of the element/or compound to one-twelfth the mass of the atom of carbon-12. As an element in nature is generally a mixture of isotopes, the relative atomic mass is the weighted mean of the atomic masses of all atoms in a specific sample of the element, weighted by isotopic abundance. In this sense, relative atomic mass was known as atomic weight.
Mass number: The mass number of isotope is the total number of nucleons in the nucleus of each atom of the isotope. Rounding the atomic mass of isotope generally gives the total nucleon count. The neutron count can also be derived by subtracting the atomic number from the mass number.

An element has 1 predominant isotope. The actual numerical difference in between the atomic mass of the main isotope and the relative atomic mass or the standard atomic weight of the element can be very small, so that it does not affect most bulk calculations; but such type of error can be critical while considering individual atoms. For the elements with more than 1 common isotope, the difference in between the atomic mass of the most common isotope and relative atomic mass of element can be as much as half a mass unit or more .The atomic mass of the uncommon isotope can differ from relative atomic mass or the standard atomic weight by several mass units.