Geologic Time and Absolute Dating
Review: Atomic Structure
• Atom
– Basic unit of an element
– Composed of protons
and neutrons (nucleus)
surrounded by electrons
– The identity of an atom
is determined by the
number of protons the
atom has
Example: Krypton
• Krypton’s atomic
number is 36, therefore
– A neutral krypton atom
has 36 protons and 36
electrons
– If atomic number ≠ 36 ≠
krypton
– The number of neutrons
=atomic number-atomic
weight (rounded up)=48
– The number of neutrons
can vary without altering
the identity of an
atom—Isotopes
Isotopes
Stable…
• Isotopes are like
people—some are
stable; some are not
• And, like people, it’s the
unstable ones that
attract our attention the
most
Seriously unstable…
Radioactivity
• Unstable isotopes are
radioactive—their nuclei
will decay over time
• A any radioactive isotope
is called a “parent”
isotope
• The decay product is
called the “daughter”
isotope
• When an isotope decays,
they do so in one of three
ways…
• Alpha emission
– Nucleus emits two protons
and two neutrons
• Plutonium-240 decays to
uranium-236
• Beta emission
– Nucleus emits an electron
• Radium-228 decays to
Actinium-228
• Electron capture
– An atom’s nucleus captures
an electron which reacts with
a proton creating a neutron
• Carbon-11 decays to Boron-11
• In a nutshell: When the
nucleus decays, a new,
more stable isotope is
created
Electron capture
Radioactive Decay and
Popcorn…yummy
• Radioactive decay is a
spontaneous and
irreversible process
• Ex. popcorn
If the Decay of an Atom Occurs Randomly, How
is it Useful to Us?
Sample
of actinium
Even a small sample
is composed of billions
of actinium atoms
(Ac-227)
After 22 years, exactly half of the
Atoms have decayed to thorium-227
All isotopes of actinium are unstable and will
decay over time. Since every atom has a certain
probability of decaying, on average, half of the
atoms in a given sample will decay to a (more) stable
daughter isotope over a set period of time
Actinium-227 has a half-life of 22 years
Half-lives
• We can use the half-life
of an isotope to figure
out the age of a rock
• How can we do this?
– Half-lives are constant
• Actinium-227 always has
a half-life of 22 years
– As the parent decays the
daughter accumulates
• Older samples = higher
number of daughter
isotopes
Example
• If we have a rock with 100
grams of a particular
isotope (Bob-12)
• Bob-12 decays to Joe-11
and has a half-life of 400
Million years
• How old is our sample if
only 25 grams of Bob-12
remain?
– Our sample is 800 My old
100 grams of
Bob-12
400 My
(one half-life)
50 grams
50 grams
+
Bob-12
Joe-11
= 100 g
400 My years
(now two
half-lives
have passed)
25 grams
75 grams
+
= 100 g
Bob-12
Joe-11
Good vs. Bad
• Isotopes with long half-lives are good for old
rocks
• Young materials are best dated by short lived
isotopes
Commonly Used Isotopes