1. Atomic Mass
Agenda
Review (whiteboards)
Read 4.7 & 4.8
Notes
Homework

2. 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

3. How do isotopes differ?
Number of neutrons in nucleus

4. How do ions differ? Number of electrons
Cations have lost electrons to gain positive charge
Anions have gained electrons to have negative charge

5. 146C
____ p+
____ e-
____ no 6 8

6. 94Be+2
____ p+
____ e-
____ no 4 2 5

7. 3617Cl-
____ p+
____ e-
____ no 17 18 19

8. Read section 4.7 & 4.8 w/ partner Six inch voices
Grab book
Read section 4.7 & 4.8 w/ partner
Six inch voices

9. Average Atomic Mass
date

10. 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

11. Atomic Mass Unit An amu is the mass of a single proton.
1 amu = 1.67*10-24 g

12. Determining Average Mass
Multiply Mass number by Percent abundance.
Add the answers.
Answer should be close to most abundant isotope.

13. Example
Cesium is 75% 133Cs, 20% 132Cs, 5% 134Cs. What is the average atomic mass?
133*.75
132*.20
134*.05
amu
Closest to the most abundant isotope

14. Example
Chromium has four istopes % 50Cr, 83.79% 52Cr, 9.5% 53Cr, 2.36% 54Cr. What is the average atomic mass of Cr?
50*.0435
52*.8379
53*.095
54*.0236
amu

15. Determining % abundance
Create an equation equaling average atomic mass
Mass1 * %X + Mass2 * %Y = average atomic mass
Create an equation equaling 100%
X + Y = 1
Use substitution to solve for x.
Mass1 * %X + Mass2 * %(1-X) = average atomic mass

16. Let one isotope be X and the other 1-X 10.013X + 11.009(1-x) = 10.812
The average atomic weight of boron is amu. Boron has two isotopes: boron-10 with a mass of and boron-11 with a mass of What are the % abundances of each isotope?
Let one isotope be X and the other 1-X
10.013X (1-x) =
10.013X – X =
0.197 = 0.996X
= X
Boron-10 is 19.78% abundant
Boron-11 is 80.22% abundant

17. What are the % abundances of the two isotopes of rubidium
What are the % abundances of the two isotopes of rubidium? Average atomic mass amu
Rb-85, amu
Rb-87, amu
Let X be for Rb-85 and 1-X for Rb-87
X (1-X) =
X – X =
= X
= X
Rb-85 is 75.17% abundant
Rb-87 is 24.83% abundant

18. Homework Complete isotopes and average atomic mass worksheet
Review scientists and discoveries.
Lab report due
Rough draft Wednesday
Final draft Friday