Tuesday
September 27, 2011
(Average Atomic
Mass; WS –
Average Atomic
Mass)
Bell Ringer
Tuesday, 9-27-11
When bombarded with light,
rubidium emits a light whose
frequency is 6.17 x 101 4 Hz.
Given that all
electromagnetic radiation
travels at the same speed,
(c = 3.00 x 108 m/s), calculate
the wavelength and
predict the color of the light.
C = óv
C = 3.00 x
1017
nm/s
4.86 x 102 nm = 486 nm
blue-green
Page
Date of
Notes on
Website
Date
Issued
Date
Due
Lab – Identifying Substances and
Mixtures
TBD
9/9
9/9
9/13
Lab – Physical Properties of
Matter
TBD
9/12
9/12
9/14
Lab – Chemical Properties of
Matter
TBD
9/14
9/14
9/16
WS – Categories, Properties, and
Phases of Matter
TBD
9/15
9/15
9/20
Test 1
TBD
9/16
9/16
9/16
WS – What’s in the Atom
TBD
9/19
9/19
9/20
Project – Atomic Structure
N/A
9/20
9/20
9/26
WS – The Electromagnetic
Spectrum
TBD
9/23
9/23
9/27
Quiz – Atomic Structure
TBD
9/26
9/26
9/26
Assignment Currently Open
Notebook
Late Date
to
Remediate
9/30
N/A
10/10
Announcements
Do you need to remediate
Test 1?
If you haven’t turned in your
project, it’s late!
Average Atomic Mass
The mass number of an
element is the number
listed (usually) under
the atomic symbol on a
Periodic Table tile.
Notice it is not a whole
number, but contains
decimal places.
The mass number is the
top number in isotopic
notation, and is
rounded off.
Average Atomic Mass
Why is the atomic
mass not a whole
number?
The atomic mass
number is an
average of all of
the masses of the
different isotopes
of that particular
element.
Average Atomic Mass
What is an
isotope?
Isotopes are atoms of the same
element that have different
numbers of neutrons.
Average Atomic Mass
What is the
relationship
between isotopes
and atomic mass?
The average atomic mass is the
weighted average of the atomic masses
of the naturally occurring isotopes of
an element.
Average Atomic Mass
The average atomic mass of an element depends on both
the mass and the relative abundance of each of the
element’s isotopes.
For example, naturally occurring copper consists of
69.17% copper-63, which has an atomic mass of 62.929 599
amu, and 30.83% copper-65, which has an atomic mass of
64.927 793 amu.
The average atomic mass of copper can be calculated by
multiplying the atomic mass of each isotope by its
relative abundance (expressed in decimal form) and
adding the results.
(0.6917 × 62.929 599 amu) + (0.3083 × 64.927 793 amu)
= 63.55 amu
The calculated average atomic mass of naturally
occurring copper is 63.55 amu.
Average Atomic Mass
Why is it important to know the percent
abundance of each isotope in an element?
Knowing this information allows us to
calculate the average atomic mass of an
element.
What do you get when you add up all of the
percent abundances of all of the isotopes?
100%
What information do you need to calculate
the average atomic mass of an element?
Average Atomic Mass
Sample Problem
Uranium has three naturally occurring isotopes as
follows:
uranium-234: abundance 0.005%, atomic mass 234.040947
uranium-235: abundance 0.720%, atomic mass 235.043924
uranium-238: abundance 99.275%, atomic mass
238.050784
Calculate the average atomic mass of
uranium.
Worksheet
Average Atomic Mass
The Least You Need to Know
The average atomic mass is the
weighted average of the atomic
masses of the naturally
occurring isotopes of an
element.
Isotopes are atoms of the same
element that have different
numbers of neutrons.
Homework
… complete your worksheets on
The Electromagnetic Spectrum
and Average Atomic Mass.