1. Atomic Mass How can atomic mass be found on the periodic table?
Underneath an element’s symbol
Decimal number
What’s the unit for atomic mass?
amu (atomic mass units)
What isotope are masses based upon?
Carbon-12 (6 protons and 6 neutrons)
Accepted mass is exactly 12 amu

2. Formula Mass
What’s the difference between a molecule and a formula unit?
Molecule: smallest unit of a covalent compound (all nonmetal elements)
Formula unit: smallest unit of an ionic compound (contains one metal element)

3. Formula Mass How do you calculate the mass of a compound?
Add the masses of all the atoms in the compound
For example: NaCl
Formula mass = mass of Na + mass of Cl
22.99 amu amu = amu

4. Formula Mass Examples Diphosphorus pentoxide, P2O5
2(30.97) + 5(16.00) = amu
Molecule
Calcium bromide, CaBr2
(79.90) = amu
Formula unit
Aluminum hydroxide, Al(OH)3
(16.00) + 3(1.01) = amu

5. Percent Composition
Gives the percentage by mass of each element in a compound
Found by dividing the mass of each element in a compound by the compound’s formula mass
Example: H2O
Formula mass = 2(1.01) = amu
% Hydrogen = 2(1.01) ×100=11.2%
% Oxygen = ×100=88.79%
*Check* =  100