1. Note: When doing calculations never clear your calculator.

2. Why do chemists need to know the mass of an atom?

3. The mass of both protons and neutrons is approximately: 1.67 x 10 -24 g The mass of an electron is approximately: 9.076 x 10 -28 g These masses are difficult to work with! So chemists developed a method of measuring the mass of an atom relative to the mass of a specific atomic standard.

4. The standard used is the carbon-12 isotope Chemists assigned the carbon-12 atom a mass of exactly 12 atomic mass units, 12 amu. = 12 amu - - - - - -

5. The relative atomic mass, A r, is the average mass of an atom of an element compared to a carbon-12 atom. It is a ratio so it has no units. For example: A r (H) = 1 meaning a H atom has a mass 1/12 of C-12. A r (Na) = 23 meaning a Na atom has a mass 23/12 of C-12.

6. The relative molecular mass, M r, is the average mass of a molecule compared to a carbon-12 atom. It is the sum of all the relative atomic masses, Ar. It is a RATIO so it has no units. M r (H 2 O) = 2 x A r (H) + A r (O) = (2 x 1) + 16 = 18