Isotopes and
Average Atomic Mass
Vocabulary:
1. isotope
2. percent abundance
3. average atomic mass
“Marilyn Monroe”,
Andy Warhol, 1962
• What element has 5 protons in its
nucleus?
• boron!!!
Question
• What element has 8 neutrons in its
nucleus?
• Don’t know because atoms of the
same element can have different
numbers of neutrons.
Question
• Isotopes: atoms of the same element
with different numbers of neutrons
• Almost all elements have many
different isotopes that are found in
nature.
Isotopes
• EX: A lump of carbon is
shown on the left. If
you analyzed the carbon
atoms in it you would
find three different
isotopes of carbon:
Isotopes
1. carbon-12
2. carbon-13
3. carbon-14
Remember
what these
numbers
means?
Isotopes
• How many protons,
electrons, and
neutrons are found
in each of the
isotopes?
1. carbon-12 1. 6, 6, 6
2. carbon-13 2. 6, 6, 7
3. carbon-14 3. 6, 6, 8
• If the three different
isotopes of carbon are
carbon-12, carbon-13,
and carbon-14, what
is the average mass of
carbon?
• It’s not 13! Any ideas
how that could be
true?
• There are many, many
more naturally
occurring carbon-12
atoms than there are
carbon-13 or carbon14 atoms
• To calculate the average mass of all the
atoms of an element, chemists must
determine the percent abundance of each
isotope of the element.
• Carbon’s percent abundance is:
• 98.93% carbon-12
• 1.06% carbon-13
• 0.01% carbon-14
Percent Abundance
• Carbon’s percent abundance is:
• 98.93% carbon-12
• 1.06% carbon-13
• 0.01% carbon-14
• So which isotope will have a larger effect
on the average mass of carbon atoms?
• What will the average mass be close to?
Percent Abundance
• To calculate the average atomic mass of an element,
you need to know two things: the percent abundance
of each isotope and the relative mass of each isotope.
Then you can use the following formula:
avg atomic mass = (%isotope 1 x massisotope 1) + (%isotope 2 x massisotope 2) + …
must be % as decimal!!!
Average Atomic Mass
• Let’s try carbon:
• What is the average atomic mass of carbon if the
percent abundance of each isotope of carbon is the
following: 98.93% carbon-12, 1.06% carbon-13, and
0.01% carbon-14? (Round to 2 decimal places.)
equation:
avg atomic mass = (%isotope 1 x massisotope 1) + (%isotope 2 x massisotope 2) + …
substitute:
avg atomic mass = (0.9893 x 12 amu) + 0.0106 x 13 amu) + (0.0001 x 14 amu)
solve:
avg atomic mass = 12.01 amu
Example 1
• Try this one on your own:
• Magnesium has three naturally occurring
isotopes. 78.70% of magnesium atoms exist as
magnesium-24, 10.03% exist as magnesium25, and 11.17% exist as magnesium-26. What
is the average atomic mass of
magnesium?(Round to 2 decimal places.)
Answer: 24.30 amu
Example 2