What relative atomic mass is
The standard is now in hand. What gets measured against it is the relative atomic mass.
Relative atomic mass (Ar)
How many times the mass of one atom of an element goes into 1/12 of the mass of one atom of the 12C isotope is the relative atomic mass of that element.
Ar = mass of one atom ÷ (1/12 × mass of one 12C atom)
Two ways of saying the same thing
The denominator, 1/12 × the mass of one 12C atom, is the atomic mass unit worked out on the previous page. So the formula can also be written like this — which is the easier form whenever a problem hands you the value of the atomic mass unit directly.
Ar = mass of one atom ÷ atomic mass unit
It has no unit
There is a mass on the top and a mass on the bottom, so the grams cancel when you divide. A relative atomic mass is a bare number. Adding "g" to the answer is the commonest error in this section.
On the next page this is actually worked through for oxygen; it is two numbers and one division.