What is atomic mass (A) of an element ?

The average mass of an atom of an element is known as its atomic mass. Atomic mass of an element is designated as A. Since, atomic mass is actual mass, hence it has the unit of mass, viz, gram, kilogram, atomic mass unit, etc. The accepted unit for describing atomic mass is atomic mass unit. The average mass of an atom of an element in atomic mass unit is called its atomic mass. From eq.1 one can write,

Average mass of 1 atom of an element = Relative atomic mass of the element × (Mass of one ^12/₆ C atom in atomic mass unit / 12)

Or Atomic mass (A) of an element = A_r × 1u=A_ru

What is relative molecular mass of a substance?

The relative molecular mass (M_r) of any substance is defined as follows:

“The relative molecular mass of a substance is the average relative mass of its molecule as compared with the mass of a ¹²₆ C atom taken as 12 units”.

Relative molecular mass of a substance,

M_r = Average mass of the molecule of the substance/ 1/12 ×(Mass of an atom of ¹²₆ C)

The relative molecular mass (Mr) is a pure number and has no unit.