Lesson 7 of 12 · 7 min
Atomic, molecular and formula masses
NCERT § "Atomic and Molecular Masses"
One water molecule has a mass of about 3 × 10⁻²³ g. No balance can weigh that, so how does anyone know that water is 18 on the mass scale?
The lesson in notes
In short
Atomic masses are measured relative to the carbon-12 isotope, whose mass is fixed at exactly 12 atomic mass units.
One atomic mass unit equals one-twelfth of a carbon-12 atom's mass, 1.66056 × 10⁻²⁴ g; it is now written u, the unified mass.
Most elements occur as a mixture of isotopes, so the atomic mass used in calculations is an average weighted by the natural abundance of each isotope; this is why carbon is taken as 12.011 u.
Molecular mass is the sum of the atomic masses of all atoms in a molecule, e.g. CH₄ = 12.011 + 4(1.008) = 16.043 u.
Ionic solids such as NaCl are three-dimensional arrays of ions, not discrete molecules, so their formula mass is used instead: NaCl = 23.0 + 35.5 = 58.5 u.
Isotope-averaging is why most tabulated atomic masses are not whole numbers.