Read 4.7 & 4.8
Parts of Atomic Theory
Atoms of the same element are identical
Atoms can rearrange – reactions
Atoms can mix in whole number ratios – compounds
Atoms are indivisible – no subatomic particles
according to Dalton
How do isotopes differ?
Number of neutrons in nucleus
How do ions differ?
Number of electrons
Cations have lost electrons to gain positive charge
Anions have gained electrons to have negative charge
____ p+ 6
____ e- 6
____ no 8
____ p+ 4
____ e- 2
____ no 5
36 Cl -
____ p+ 17
____ e- 18
____ no 19
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?
Closest to the most abundant isotope
Chromium has four istopes. 4.35%
50Cr, 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?
Let one isotope be X and the other 1-X
10.013X + 11.009(1-x) = 10.812
10.013X + 11.009 – 11.009X = 10.812
0.197 = 0.996X
0.1978 = X
Boron-10 is 19.78% abundant
Boron-11 is 80.22% abundant
What are the % abundances of the two
isotopes of rubidium? Average atomic
mass 85.4678 amu
Rb-85, 84.9918 amu
Rb-87, 86.9092 amu
Let X be for Rb-85 and 1-X for Rb-87
84.9918X + 86.9092(1-X) = 85.4678
84.9918X + 86.9092 – 86.9092X = 85.4678
1.4414 = 1.9174X
0.7517 = X
Rb-85 is 75.17% abundant
Rb-87 is 24.83% abundant
Complete isotopes and average atomic
Review scientists and discoveries.
Lab report due
Rough draft Wednesday
Final draft Friday