Question

What was the need for using relative atomic mass or relative molecular mass?

• A. To round off every atomic mass as a multiple of 2
• B. To create new terminologies
• C. As the mass of atoms or molecules were small
• D. None of the above

Answer 1

Answer: (C)

As the mass of atoms or molecules were small

Relative atomic mass or relative molecular mass was used as the mass of the atoms or molecules was very small. Therefore there was a need to be taken relative to some other elements.

The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes - because if there is much more of one isotope then that will influence the average mass much more than the less abundant isotope will.

Relative atomic mass

Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12.

Atoms with an Ar of less than this have a smaller mass than a carbon atom.

Atoms with an Ar that is more than this have a larger mass than a carbon atom.

Chlorine's Ar of 35.5 is an average of the masses of the different isotopes of chlorine.