Atomic Theory
The Average Atomic Weight
A Quick Review
• We have covered the
structure of the atom.
– It has a nucleus where
the protons and
neutrons are located
– It has an electron cloud
where the electrons
exist
• We have the following
definitions:
– Atomic Number: tells us
the number of protons
in the atom’s nucleus
– Mass Number: tells us
the sum of the protons +
neutrons in a given
nucleus of a specific
isotope.
First New Concept
• Chemistry has invented a
unit of mass for atoms.
• It is called the “atomic
mass unit” or “amu” .
• You should know that
both the proton and the
neutron have a mass of 1
amu.
• Each proton “weighs” 1
amu.
• So does each neutron.
• Therefore, if an atom has
8 protons and 8 neutrons,
its mass will be described
as 16 amu.
• Important to know that
the mass of electrons is
essentially “zero” – they
are so small that we
ignore their mass in this
calculation.
Finishing this 1st Concept
• If we look back at the
previous slide, we see
that the mass of that
atom was calculated by
adding the number of
protons and neutrons
together and calling the
result the mass in
amu’s.
• We already know that
the sum of protons +
neutrons is called the
Mass Number.
• Therefore, we can
simply state that the
mass of any isotope is
simply that atom’s mass
number in amu’s.
Part II – The Other Number on the
Periodic Chart
17
________________________
Cl
________________________
35.453
________________________
Consider this:
17
Cl
35.453
The element Chlorine
exists as two isotopes.
• Chlorine – 35 which
weighs 35 amu
• Chlorine – 36 which
weighs 36 amu
The question to be
asked is where does
the 35.453 come from?
The Answer:
• The answer to the
question on the
previous slide is that
the atomic weight that
appears on the periodic
chart is actually a
“weighted average”.
• It takes into account
two different factors:
– The actual weight of
the individual
isotopes
– What percentage of
the entire element is
the individual
isotope.
Consider the Following Scenarios
90
90
90
90
90
90
90
90
90
90
What would the average score on this test be?
Now This One:
90
90
90
90
90
80
80
80
80
80
What about the average of this test
Consider this set:
90
80
90
90
90
80
90
90
90
80
Back to the Chlorine
17
Cl
35.453
If Chlorine exists as Cl – 35 and
Cl – 36 and the average atomic
weight is the 35.453 , which
statement is correct?
a. There is more Cl – 35 than
there is Cl – 36.
b. The Cl – 35 and the Cl – 36
exist equally.
c. There is more Cl – 36 than
there is Cl – 35.
State Expectations
5
B
10.811
1. Looking at the atomic
weight, what two
isotopes are likely for
this element?
2. Based on your
answer to #1, which
of the isotopes is
more common?
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