1. Section 2.4
Atomic Weights

2. Atomic Weight
Determined by average atomic masses of the isotopes of an element
Atomic Weight = Sum of multiplying the abundance of each isotope by its atomic mass

3. Isotopes: There are 23 known isotopes of krypton, 6 stable and 17 unstable. Natural krypton contains the 6 stable isotopes.

4. Sample Atomic Weight Problem
Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass of amu, and 24.22% 37Cl, which has an atomic mass of amu. Calculate the average atomic weight of chlorine.

5. Solution Average atomic weight =
(0.7578)( amu) + (0.2422)( amu)
= amu
Don’t forget your Sig Figs!!!