Download 1.3 Understanding Atomic Mass

1.3 Understanding Atomic Mass
A Review: Subatomic particles
Name
Symbol
Relative
Charge mass
Actual
mass (g)
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
no
0
1
1.67 x 10-24
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Structure of the Atom
There are two regions:
1. The Nucleus:
• With protons and neutrons
• Positive charge
• Almost all the mass
2. Electron cloud
• Most of the volume of an atom
• The region where the electrons can be found
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http://www.andrew.cmu.edu/user/jgrao/Manchester%20United/giant%20stadium.jpg
If the atom was the size of a stadium,
the nucleus would be the size of:
a marble.
http://www.akronmarbles.com/marble-frag-2.jpg
Standard Atomic Notation
Includes the symbol of the element, the
mass number and the atomic number
# protons
+ # neutrons
mass number
# protons
Mass
number
Atomic
number
X
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http://chemistry.beloit.edu/Stars/EMSpectrum/fluorine.gif
• Find the
– number of protons = 9 +
– number of neutrons = 10
– number of electrons = 9
– Atomic number = 9
– Mass number = 19
19
9
F
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Find the
– number of protons = 35
– number of neutrons = 45
– number of electrons = 35
– Atomic number = 35
– Mass number = 80
80
35
Br
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Find the
– number of protons = 11
– number of neutrons = 12
– number of electrons = 10
– Atomic number = 11
– Mass number = 23
23
11
1+
Na
Sodium ion
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http://www.webelements.com/webelements/elements/media/nearingzero/Na.gif
http://discover.edventures.com/images/termlib/i/isotope/support.gif
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
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
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 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?
http://discover.edventures.com/images/termlib/i/isotope/support.gif
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
based on the abundance of each naturally
occurring isotope of that element.
• The Average Atomic Mass is shown on
Periodic Tables.
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.
Radioactive Isotopes
• Radioactive:
– Capable of spontaneously emitting radiation in
the form of particles and/or gamma rays
• Radioisotope:
– A radioactive isotope of an element that occurs
naturally or is produced artificially.
Radioisotopes
• Many elements have one or more isotopes
that are unstable.
• Atoms of unstable isotopes decay, emitting
radiation as their nucleus changes.
• The changes can happen quickly or slowly,
depending on the isotope.
• Radiation emitted can be harmless or very
dangerous to living cells.
Applications of Nuclear Science
Anthropology:
Radiology:
Nuclear Reactors:
http://www.johnharveyphoto.com/RoadTrip/NuclearReactor.jpg
Radioisotopes
• Radioactive isotopes
• Many uses
– Medical diagnostics
– Optimal composition of
fertilizers
– Abrasion studies in engines
and tires
Radioisotope is injected
into the bloodstream to
observe circulation.
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