Relative masses of atoms and molecules

Need to know

What you need to know

• The standard for relative atomic mass is one atom of carbon-12.
• It is defined as having a mass of exactly 12.
• One 'atomic mass unit' (amu) is defined as 1/12th of the mass of a carbon-12 atom.

Explanation

Weighing the Unweighable

1. The relative atomic mass (Ar) of an element is the weighted average mass of its isotopes, found on the periodic table. For example, chlorine's Ar is 35.5 because it's a mix of different isotopes.
2. The relative molecular mass (Mr) of a molecule is the sum of the relative atomic masses of all its atoms. For H₂O, it's (2 x 1.0) + 16.0 = 18.0.
3. Relative isotopic mass is the mass of a single isotope (e.g., carbon-13), while relative atomic mass (Ar) is the average for the element. The standard for all relative masses is the carbon-12 atom, defined as exactly 12.
4. Relative mass (Mr) is essential for mole calculations, which connect the microscopic world of atoms to the macroscopic world of grams that we can measure in the lab. This is a key link to Topic 2.2.