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Day 26 Average Atomic Mass

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					Atomic Mass
                      Agenda
 Review (whiteboards)
 Read 4.7 & 4.8
 Notes
 Homework
Parts of Atomic Theory
How do isotopes differ?
How do ions differ?
14
     6C
 ____ p+
 ____ e-
 ____ no
9       Be+2
    4
 ____ p+
 ____ e-
 ____ no
36        Cl -
     17
 ____ p+
 ____ e-
 ____ no
READ
Grab book
Read section 4.7 & 4.8 w/
 partner
Six inch voices
Average Atomic Mass
          date
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
  abundance.
 Add the answers.
 Answer should be close to most
  abundant isotope.
Example
 Cesium is 75% 133Cs, 20% 132Cs, 5% 134Cs.
 What is the average atomic mass?
Example
 Chromium has four istopes. 4.35%
    83.79% 52Cr, 9.5% 53Cr, 2.36% 54Cr.
 50Cr,

 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%
    X+Y=1
 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
Homework
 Complete isotopes and average atomic
  mass worksheet
 Review scientists and discoveries.
 Lab report due
   Rough draft Wednesday
   Final draft Friday

				
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posted:10/27/2011
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