# 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
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.
 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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 views: 10 posted: 10/27/2011 language: English pages: 18