Relative atomic mass, , is the weighted mean mass of an atom of an element, whereas relative formula mass, , is the total relative mass of all atoms represented in a compound's formula. Both are relative quantities and therefore have no units.
Relative atomic mass, , is the weighted mean mass of an atom of an element, whereas relative formula mass, , is the total relative mass of all atoms represented in a compound's formula. Both are relative quantities and therefore have no units.
This distinction is part of Structure 1.4, Counting particles by mass, and applies at both SL and HL.
| Quantity | Meaning | How it is found |
|---|---|---|
| Relative atomic mass, | Weighted mean mass of an atom of an element compared with of the mass of a carbon-12 atom | Use isotopic masses and their relative abundances, or read the value from the periodic table |
| Relative formula mass, | Relative mass of one formula unit, calculated from all atoms shown in its chemical formula | Add the values of every atom in the formula |
An element's is usually not a whole number because most elements occur as mixtures of isotopes. For example, chlorine's isotopic composition gives it an of approximately .
Using simplified abundances of 75.8% chlorine-35 and 24.2% chlorine-37:
This is a weighted mean across many chlorine atoms, not the mass of one particular chlorine atom. The decimal therefore reflects isotope abundance rather than a fractional number of particles.
For magnesium chloride, :
Therefore, the relative formula mass of is , with no unit. The subscript 2 is essential because the formula contains two chlorine atoms for every magnesium atom.
A common misconception is that has units of . It does not. Molar mass, , has the same numerical value but carries units: the molar mass of is .
When asked to “calculate” , show each , multiply by the number of atoms, and add the results. Check brackets and subscripts carefully, and do not attach units unless the question asks for molar mass rather than relative formula mass.