Transcript Atomic Mass of Chlorine (35.45amu)

Unit 3: Atomic
Structure
Atomic Mass Units (amu) and
Calculating Atomic Mass
Remember???
• Mass number = protons + neutrons
• Isotopes= atoms with different # of
neutrons
• Atomic masses occur with decimal
points because they are an average of
different isotopes
Atomic Mass Units
• Defined as 1/12 the mass of a
Carbon-12 atom.
• The way that atomic mass is
measured in the Periodic Table
• Abbreviated “amu”
To find the atomic mass,
you need to know:
• Number of stable
isotopes
• Mass of each isotope
• Natural % abundance
of each isotope
Real-World Application:
Weighted Average
Let’s just say that the weights that make up
your grade are as follows:
• Tests – 70% of grade
• Quizzes – 20 % of grade
• Homework –10% of grade
You earn the following grades:
test – 80, quiz – 100, homework – 100
To Find Weighted Average…
Final Grade =
(.7)(80)+(.2)(100)+(.1)(100)
56 + 20 + 10
= 86%
Don’t forget!
All percentages must = 100%
All decimals = 1
To Find Atomic Mass :
Atomic mass of element =
A.M. isotope1*(%1) + A.M. of isotope2 *(%2)
100
100
Example - Chlorine
Calculate the atomic mass of
chlorine if the two common
isotopes of chlorine have masses
of 35.45 amu (75.00% abundance)
and 37.29 amu (25.00%
abundance).
Atomic Mass of Chlorine
a.m.= 35.45amu
%1= 75.00%
a.m.=37.29amu
%2= 25.00%
(35.45amu)(.7500) + (37.29amu)(.2500)
26.59 amu + 9.323 amu
Atomic mass of Chlorine= 35.91 amu
What if you don’t have the percentage
or the atomic mass?
Calculate the atomic mass of one of
indium’s isotopes (there are two) if the
atomic mass of indium is 114.8200
amu and one isotope of indium has a
mass of 114.9041 amu and an
abundance of 95.795%.
a.m. of indium=114.8200amu
a.m.= 114.9041amu a.m.= ?
%1= 95.795%
%2= ?100-95.795= 4.205%
114.8200amu = (114.9041amu)*(0.95795)+(x)*(0.04205)
114.8200amu = 110.07 + (x)*(0.04205)
-110.07
-110.07
4.75 = 0.04205x
0.04205 0.04205
Atomic mass of Indium= 113 amu