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Unit 3
What is an isotope?
Atoms of the same element can have
different numbers of neutrons; the different
possible versions of each element are called
isotopes.
 For example, the most common isotope of
hydrogen has no neutrons. There is another
version of hydrogen with 1 neutron, and another
version with 2 neutrons.
Hydrogen-1
Hydrogen-2
Hydrogen - 3
What makes one isotope different from
another isotope?
__________________________________
Symbols for Isotopes
I Do: Write the isotope symbol and name for
an atom with 42 protons, 40 electrons, and
54 neutrons
Symbol______Name__________________
We Do: Write the isotope symbol and name for an
atom with 42 protons, 42 electrons, and 58
neutrons
Symbol______Name________________________
You Do: Write the isotope symbol and name for an
atom with 50 protons, 50 electrons, and 62
neutrons
Symbol______Name________________________
How many isotopes can one element
have? Can an atom have just any
number of neutrons?
There ARE "preferred" combinations of
neutrons and protons
– Light elements tend to have about as many
neutrons as protons
– Heavy elements apparently need more
neutrons than protons in order to stick
together.
– Atoms with a few too many neutrons, or not
quite enough, can sometimes exist for a
while, but they're unstable.
How many isotopes can one element
have? Can an atom have just any
number of neutrons?
• Do atoms just fall apart if they don't have
the right number of neutrons?
– YES! Unstable atoms are radioactive: their
nuclei change or decay by spitting out
radiation, in the form of particles or
electromagnetic waves.
Common Uses for Isotopes
Smoke detectors - Many contain a small
amount of americium-241. By utilizing the
radioactive properties of this material,
smoke from a fire can be detected at a
very early stage.
Common Uses for Isotopes
Radioactive dating - If the half life of a given
isotope (usually Carbon-14) is known, the
amount of that isotope and the product of its
decay can be analyzed to determine the age of
once-living organisms
Example : If an organism such as a tree contained
1 gram of carbon-14 while it was living, then
after 5700 years it would contain half that
amount, or 0.5 grams of carbon-14.
Common Uses for Isotopes
Medicine - Radioactive isotopes can be
injected into a patient’s body. These
isotopes will then appear on a scan and
can help doctors diagnose problems or
disease
Common Uses for Isotopes
Nuclear Energy - Radioactive elements such
as uranium can be utilized to create
energy through fission (splitting apart).
Average Atomic Mass
Listed on the periodic table
• Is a weighted average of all the isotopes
for that element.
• This is why atomic mass is not a whole
number.
Steps for Determining Average
Atomic Weights
1.
2.
3.
4.
Turn the percent into a decimal.
Multiply the decimal by the mass.
Repeat for each isotope
Add the masses up.
Example: Magnesium
Isotopes
Mass
of Isotope
Percent
Abundance
Mg-24 =
24.0 amu
78.70%
= _____________
Mg-25 =
25.0 amu
10.13%
= _____________
Mg-26 =
26.0 amu
11.17%
= _____________
Atomic mass (average mass) Mg= ___________ amu