Average Atomic Mass An element can exist in a number of forms called isotopes Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons Isotopes ID: 741511
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Slide1
Calculations in Chemistry:
Average Atomic MassSlide2
An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in
mass as a result of a different number of neutrons.
IsotopesSlide3
T
here
are two naturally occurring isotopes of
copper.One isotope weighs in at 62.93
amu
, the other has a mass of 64.94 amu. (atomic mass units)These two isotopes have different proportions in a natural sample of copper.The lighter isotope is more common with an abundance of 69.09% of all naturally occurring copper. What is the percent abundance of the other isotope?
Isotopes of CopperSlide4
The remainder of the atoms, 30.91 %, have a mass of 64.94 amu
. Slide5
The calculation of the average atomic mass is a WEIGHTED AVERAGE
.
To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type
.
Average Atomic MassSlide6
Average atomic mass
=
Σ (mass of isotope × relative abundance)
Σ = sum or to add them all together
Calculation of Average MassSlide7
Since there are two
isotopes for copper, we
will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in)
The relative abundance is simply the percentage of the isotope, but in decimal form.
69.09
% corresponds to a relative abundance of 0.6909. Calculation of Average MassSlide8
Average atomic mass of copper
=
(62.93 amu × 0.6909)
[the mass and abundance of isotope #1]
+(64.94 amu × 0.3091) [the mass and abundance of isotope #2]=63.55Calculation of Average MassSlide9
An
AVERAGE atom of copper has a mass of
63.55 amu.
Notice that in this problem, we could predict that the average is closer to the weight of the lighter isotope
. Why?
The lighter form of copper is more abundant.Calculation of Average MassSlide10
Isotope name
Isotope mass (amu)
Relative abundance
Silicon – 28
27.98
92.21%
Silicon – 29
28.98
4.70%
Silicon – 30
29.97
3.09%
Practice
Isotope name
Isotope mass (amu)
Percent abundance
Silver-107
106.90509
51.86
Silver-109
108.90470remainder
Isotope name
Isotope mass (amu)
percentage
Iron – 54
53.94
5.90%
Iron – 56
55.93
91.72%
Iron – 57
56.94
2.10%
Iron – 58
57.93
0.280%Slide11
Silver = 107.87 amu
Silicon = 28.09
amuIron = 55.84
amu
Check Your AnswersSlide12
Read section 1.4 in the text to support what we did in class today.
pg
27 #1Pg 29 #1-5,9
Homework