1. Mass of Individual Atoms
Review:
Mass of proton = x g
Mass of neutron = ?
Mass of electron = 9.11 x g
An atom of K-39, has ____ protons, _____ neutrons, and ______ electrons.
Who wants to add up that actual mass?!?!

2. Mass of Individual Atoms (cont.)
We ALL want to work with whole numbers, so scientists came up with the atomic mass unit (amu).
Def.: 1/12 the mass of a carbon-12 atom (ALMOST equal to the mass of 1 proton or 1 neutron)

3. Mass of Individual Atoms (cont.)
Review: What does the mass number of an element tell you?
The mass recorded on the periodic table for an element is called atomic mass.
Atomic mass is determined by the weighted average mass of the isotopes of that element.

4. Mass of Individual Atoms (cont.)
What is the atomic mass for chlorine?
Chlorine exists naturally as a mixture of % Cl-35 and % Cl-37.
What does Cl-35 and Cl-37 mean?
The ACTUAL masses of these isotopes would be amu and amu.
The math:
(75.770% x ) + (24.230% x ) = amu
What was the atomic mass of chlorine? 

5. Practice & Assignment To calculate atomic mass: P.104: 15-16
Calculate mass contribution of each isotope by multiplying its mass x its % abundance
Add together all the mass contributions.
P.104: 15-16
Study Guide Workbook: pp (24-33)