1. Chapter 6 Chemical Composition
Calculating the masses and composition of atoms and compounds.

2. Atomic Masses
Historically individual atoms were far too small to be accurately weighed.
The mass of atoms was initially based on a relative scale.
A relative scale measures one quantity of a substance based on the comparison to another substance.

3. The AMU History
The first standard for comparison for the mass of an atom was based on the lightest element, Hydrogen.
An AMU (atomic mass unit) was based on this mass. (essentially meaning the amu is equal to the mass of a proton)
Next after discovering that hydrogen has 3 isotopes Oxygen was chosen as the standard and the amu was defined as 1/16th the mass of oxygen.
Again with the discovery of Oxygen’s isotopes a new standard was chosen.
This time however the standard was based on a specific isotope.

4. The AMU = C-12
Finally the C-12 isotope was chosen as the standard of comparison for the amu.
C-12 has 6 protons and 6 neutrons.
The amu = 1/12th the mass of C-12
Realistically then the mass of the amu hasn’t changed, it is still relatively the mass of a single proton or neutron.

5. Atomic Masses in AMU
The value for atomic mass listed on the Periodic Table is the mass of the element in amu.
Historically it was discovered to have a mass equal to 1.66 x grams.
We can use this to calculate the mass of individual atoms.

6. Counting by Weighing
Conversely, the number of atoms in a measured sample can be determined if the mass of any individual atom is known.
The mass can be given in amu or in grams.
Dimensional Analysis is used to convert the mass to number of atoms.