Download average

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# protons
+ # neutrons
mass number
# protons
Mass
number
Atomic
number
X
Isotopes
• Atoms of the same element can have
different numbers of neutrons
This gives them different mass
numbers.
They have the same atomic numbers.
They look, act and react the same.
C-12
vs.
C-14
Naming Isotopes
• Put the mass number after the name of
the element
• Examples:
• Carbon-12 and Carbon-14
• Uranium-235 and Uranium-238
Isotopes of Hydrogen
• Protium (H-1)
1 proton, 0 neutrons, 1 electron
Most abundant isotope
• Deuterium (H-2)
1 proton, 1 neutron, 1 electron
Used in “heavy water”
• Tritium (H-3)
1 proton, 2 neutrons, 1 electron
radioactive
Isotopes of Three Common Elements
Mass
Element
Carbon
Chlorine
Silicon
Symbol
Mass (amu)
Fractional
Abundance
Number
12
6
C
12
12 (exactly)
99.89%
13
6
C
13
13.003
1.11%
35
17
Cl
35
34.969
75.53%
37
17
Cl
37
36.966
24.47%
28
29
30
27.977
28.976
29.974
28
14
29
14
Si
Si
30
Si
14
LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World , 1996, page 110
92.21%
4.70%
3.09%
Average
Atomic
Mass
12.01
35.45
28.09
Which isotopes are represented in this sketch?
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Calculations of
Average Atomic Mass
Why is the mass number on a
periodic table never a nice whole
number?
Mass Number vs
AverageAtomic Mass
• The Mass Number is specific to one
isotope.
• The Average Atomic Mass is the
calculated mass based on the abundance
of each naturally occurring isotope of that
element.
• The Average Atomic Mass is shown on
Periodic Tables. It is the most accurate
number that should be used in
calculations.
Average Atomic Mass
• How heavy is an atom of oxygen?
• There are different kinds of oxygen atoms.
oxygen-16,
oxygen-17 and
oxygen-18
• We are more concerned with average atomic
mass, based on the abundance of each element in
nature.
• We don’t use grams because the numbers would
be too small.
California WEB
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Atomic Mass Unit
(amu or u)
• This unit is used instead of grams.
• It is 1/12 of the mass of a carbon-12 atom.
Your book shows this a different
way
• Experts show that chlorine is a mixture of
75.77% Cl-35 and 24.33% of Cl-37. If the
precise molar mass of Cl-35 is
34.968 852g and Cl – 37 is 36. 965 903
grams what is the average molar mass of
the chlorine atoms in the mixture?
• Assume 100 moles of atoms are used in
mixture
• Calculate the number of moles of each
isotope
• Calculate the mass of each isotope
• Add the masses together to get the total
mass of the mixture
• # moles of Cl-35 = 0.7577 x 100 mol = 75.77 mol
of Cl-35
– 75.77 mol X 34.968 852 g/mol = 26.4959g of Cl-35
• # moles of Cl-37 = 0.2423 x 100 mol = 24.23 mol
– 24.23 mol X 36.965 903 g/mol = 8.9568 g of Cl-37
Total mass of mixture = mass of Cl-35 +
Cl-37 = 35.453g