The atomic mass unit
Saying "relative to it" means deciding relative to what. The standard chosen for that job is called the atomic mass unit.
The atomic mass unit
Whatever the masses of atoms are expressed relative to is called the atomic mass unit.
What has been chosen?
In the past the mass of a hydrogen atom, the lightest element, was used for this. Today the atomic mass unit is 1/12 of the mass of one atom of the 12C isotope.
How much is that?
atomic mass unit = (mass of one 12C atom) ÷ 12
= (1.99 × 10-23 g) ÷ 12
= 1.66 × 10-24 g
You will meet 1.67 as well
Books quote this value as either 1.66 × 10-24 g or 1.67 × 10-24 g, depending on how far the mass of the carbon atom was rounded before dividing. Some problems later in this chapter hand you 1.67. Use whichever value the question gives you — after rounding, both lead to the same relative atomic mass.
Why a twelfth in particular?
The nucleus of a carbon-12 atom holds six protons and six neutrons — twelve particles. Take a twelfth of its mass and what you get is close to the mass of one of those particles. That is what makes the relative atomic mass of almost every element come out near a whole number.
This one number carries the whole chapter
This one small mass is the denominator that keeps returning over the next several pages. Divide the mass of any atom or molecule by it and what comes out is a number with no unit — which is exactly what we are after.