Read 4.7 & 4.8
Parts of Atomic Theory
How do isotopes differ?
How do ions differ?
36 Cl -
Read section 4.7 & 4.8 w/
Six inch voices
Average Atomic Mass
Average atomic mass
A WEIGHTED average dependent on the percent
abundance of each isotope.
If all isotopes were in a bag, how frequently would you
pull out each separate isotope.
Measured in amu’s
Atomic Mass Unit
An amu is the mass of a single proton.
1 amu = 1.67*10-24 g
Determining Average Mass
Multiply Mass number by Percent
Add the answers.
Answer should be close to most
Cesium is 75% 133Cs, 20% 132Cs, 5% 134Cs.
What is the average atomic mass?
Chromium has four istopes. 4.35%
83.79% 52Cr, 9.5% 53Cr, 2.36% 54Cr.
What is the average atomic mass of Cr?
Determining % abundance
Create an equation equaling average atomic mass
Mass1 * %X + Mass2 * %Y = average atomic mass
Create an equation equaling 100%
Use substitution to solve for x.
Mass1 * %X + Mass2 * %(1-X) = average atomic mass
The average atomic weight of boron is 10.812amu. Boron has two
isotopes: boron-10 with a mass of 10.013 and boron-11 with a
mass of 11.009. What are the % abundances of each isotope?
What are the % abundances of the
two isotopes of rubidium?
Rb-85, 84.9918 amu
Rb-87, 86.9092 amu
Complete isotopes and average atomic
Review scientists and discoveries.
Lab report due
Rough draft Wednesday
Final draft Friday