1. Atomic Weight What does it mean?

2. The Mass of an Atom The mass of an atom is measured in atomic mass units (amu) aka Dalton. The atomic mass of an atom is the sum of the mass of its protons and neutrons Each proton and neutron weigh @1 amu By international agreement the Carbon-12 isotope (6 protons + 6 neutrons) is the standard by which every other type of atom is measured.

3. Atomic Weight (Mass) You will notice each element’s mass in the periodic table is NOT an even number. Atomic weight is actually the ratioed average mass of all the known isotopes of each element. The proportions are based on each isotope’s natural abundance. Ex: the atomic mass of Carbon is 12.011amu (atomic mass units) The majority of carbon isotopes are Carbon-12, but Carbon-13 and Carbon-14 exist in the same proportion everywhere. The average atomic mass of carbon is 12.011

4. Formula Weight A formula tells us: Which elements are present How many of each element is present An ionic compound does not exist as itself, but as part of a matrix They are not expressed as molecules but as a formula unit The sum of the atomic weights of a formula unit of an ionic compound gives the formula weight Just Add up the atomic mass of each type!

5. Examples NaCl Na = 22.98amu Cl = 35.45amu NaCl = 58.43amu NH 4 NO 3 N = 14.01 X 2 = 28.02 H = 1.00 x 4 = 4.00 O = 16.00 x 3 = 48.00 NH 4 NO 3 = 80.02amu

6. Molecular Weight The formula weight of one unit of a molecular compound, i.e., covalent compounds Water H 2 O H: 2 atoms @ 1.00amu + O: 1 atom @ 16.00amu = 18.00amu Octane: C 8 H 18 C = 8 x 12.01 = 96.08amu H = 18 x 1.00 = 18.00amu C 8 H 18 = 114.08amu

7. Assignment Atomic, Formula, and Molecular weight problem set Due Tuesday Jan 13.