CBSE Class 9 Answered

An atomic weight (relative atomic mass) of an element from a specified source is the ratio of the average mass per atom of the element to 1/12 of the mass of ¹²C" in its nuclear and electronic ground state.

A sample of any element consists of one or more isotopes of that element. Each isotope is a different weight. The relative amounts of each isotope for any element represents the isotope distribution for that element. The atomic weight is the average of the isotope weights weighted for the isotope distribution and expressed on the ¹²C scale as mentioned above.

In general, add no of protons and neutrons to get atomic weight. As atomic weight = mass of protons + mass of neutrons.