1. How to calculate it!
Average Atomic Mass

2. Quiz!
Write the number of protons, neutrons and electrons for each isotope:
1. B-11
2. C-14
3. H-2
4. O-18
5. Na-23

3. Average Atomic Mass
Weighted average of all isotopes’ masses for an element
Calculate:
Percent of isotope one * mass of that isotope
PLUS
Percent of isotope two * mass of that isotope
Percent of isotope three * mass of that isotope
And so on!

4. Example
Naturally occurring copper is 69.17% copper-63 with a mass of amu and 30.83% copper-65 with a mass of amu. What is the average atomic mass of copper?
What we know:
Cu % amu
Cu % amu
Equation:
%*mass + %*mass = average mass

5. Example continued Substitute: Solve!
(0.6917)(62.929) + (0.3083)(64.929) = avg mass
Solve!
amu amu = amu

6. Radioactive Isotopes
Some isotopes have too many particles in the nucleus
Radioactive decay: nucleus breaks, forming a new element
May take many decay steps to reach a stable isotope
Time it takes half the atoms to decay is called half life

7. Problem solving activity p 552
Radioactive Element
Changes to Element
Half-life
U-238
Pb-206
4 460 my
K-40
Ar-40
1 260 my
Rb-87
Sr-87 my
C-14
N-14
5 730 years
How many years would it take for half the Rb-87 atoms in a piece of rock to decay?
How long would it take for 75% of the atoms to decay?

8. Problem solving activity p 552
Radioactive Element
Changes to Element
Half-life
U-238
Pb-206
4 460 my
K-40
Ar-40
1 260 my
Rb-87
Sr-87 my
C-14
N-14
5 730 years
After a long time only 25% of the atoms of U-238 remain. How old would you predict the rock is?
What element are the other 75% of the atoms now?