Chapter 16, Lesson 3, covers the concept of absolute age dating in geology, explaining how scientists determine the numerical age of rocks using isotopes and radioactive decay. The chapter discusses key terms such as isotopes, half-lives, and radiocarbon dating, highlighting the processes and methods used to date organic and inorganic materials. It also provides insights on the age of Earth and various isotopes applicable for dating different types of rocks.

1. Chapter 16 Lesson 3: Absolute Age Dating – p582–589 – page 1
Vocabulary
 Absolute age (583) – the numerical age, in years, of a rock or object
 Isotope (584) – atoms of the same element that have different numbers of neutrons
 Radioactive decay (584) – the process by which an unstable element naturally changes into
another element that is stable
 Half-life (585) – the time required for half of the parent isotopes to decay into daughter
isotopes
Absolute Age of Rocks
 Absolute age means the ______________________ age, in years, of a rock or object.
o What is your absolute age?
o How is absolute age different from relative age?
 Scientists have been able to determine the absolute ages of rocks and other objects only
since the beginning of the ___________________________ century.
o Once ______________________________ had been discovered.
o Radioactivity is the release of _________________________ from unstable atoms.
Atoms
 You are all familiar with atoms.
o What are the parts of an atom?
o What determines the element of an atom?
o What is in the nucleus of an atom?
o What surrounds the nucleus?
Review of Isotopes
 All atoms of a given element have the same number of _______________________.
o How many protons does a hydrogen atom have?
 However, an element’s atoms can have different numbers of_____________________.
 Atoms of the same element that have different numbers of neutrons are called isotopes.
o We name isotopes with the element name and the number of particles
(protons+neutrons) in its nucleus.

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Radioactive Decay
 Most isotopes are________________________________.
o Stable isotopes do not change under normal conditions
 Unstable isotopes are called _______________________________ isotopes.
o Radioactive isotopes _______________________, or change, over time.
o As they decay, they release energy and form _______________, stable atoms.
 Radioactive decay is the process by which an unstable element naturally changes into
another element that is ________________________.
 The unstable isotope that decays is called the ______________________ isotope.
 The new element that forms is called the ____________________________ isotope.
o In the figure, the atoms of an unstable isotope of hydrogen (parent) decay into
atoms of a stable isotope of helium (__________________________)
Half-Life
 The rate of decay from parent isotopes into daughter isotopes is different for
_________________________ radioactive elements.
o Rate of decay is constant for a given __________________________________
o This rate is measure in _______________________ units called half-lives

3. Chapter 16 Lesson 3: Absolute Age Dating – p582–589 – page 3
 An isotope’s half-life is the time required for ____________________
of the parent isotopes to decay into daughter isotopes.
o Half-lives of radioactive isotopes range from a few
microseconds to _______________________ of years.
 As time passes, more and more unstable parent isotopes decay and
form _______________________ daughter isotopes.
 The means the ratio of parent and daughter isotopes is always
________________________.
 When ______________________ the parent isotopes have decayed
into daughter isotopes, the isotope has reached one half-life.
Half-lives Time
Percentageofremaining
parentatoms
0 1 2 3 4
100
50
25
12.5
6.25
 After one half-life, _________% of the isotopes are parents and _________% of the
isotopes are daughters
 After two half-lives, 50% of the remaining parent isotopes have decayed so that only
___________% of the original parent isotopes remain.
 This process continues until nearly all parent isotopes have decayed into daughter
isotopes.
Radiometric Ages
 Because radioactive isotopes decay at a constant rate, they can be used like
_____________________ to measure the age of the material that contains them.

4. Chapter 16 Lesson 3: Absolute Age Dating – p582–589 – page 4
 In this process, called _________________ ____________________, scientists measure
the amount of parent isotope and daughter isotope in a sample of material they want to
date.
o From this ratio, they can determine the material’s _________________.
Review
 What is measured in radiometric dating?
Radiocarbon Dating
 One important radioactive isotope used for dating is an isotope of carbon called
_____________________________.
o Radiocarbon is also known as carbon-14 or C-14.
 How many protons and neutrons does C-14 have?
 Radiocarbon forms in Earth’s upper atmosphere where it mixes with a stable carbon
isotope called carbon-_______ or C-12.
 The ratio of the amount of C-14 and C-12 in the _________________________ is constant.
 All _____________________ things use carbon as they build and repair tissues
 As long as an organism is alive, the ratio of C-14 to C-12 in its tissues is
______________________ to the ratio in the atmosphere.
 However, if an organism dies, it stops taking in C-______.
o The C-14 present in the organism starts to decay to _________________-14 (N-14).
o As the dead organism’s C-14 decays, the ratio of C-14 to C-12
_____________________.
 Scientists measure the ratio of C-14 to C-12 in the remains of the dead organism to
determine how much time has passed since the organism died.
o The half-life of carbon-14 is _________________________ years.
 That means radiocarbon dating is useful for measuring the age of remains of organisms
that died up to about _________________________ years ago.
o In remains older than this, there is not enough _____________ left to measure
accurately.
Review
 What two isotopes of carbon are present in our atmosphere?
 Is the ratio of carbon isotopes in the atmosphere constant or changing?
 C-14 decays into what isotope?

5. Chapter 16 Lesson 3: Absolute Age Dating – p582–589 – page 5
 Should we expect more C-14 or N-14 in an organism that has been dead for 40,000 years?
Dating Rocks
 Radiocarbon dating is useful only for dating
_____________________ material – material from
once-living organisms.
o This material includes bones, _________________,
parchment, and charcoal.
 Most rocks do not contain __________________________
material.
 Even most ________________________ are no longer
organic.
o Their living tissue has been replaced by rock-forming
_______________________.
 So, for dating rocks, geologists use different kinds of
radioactive isotopes.
Dating Igneous Rock
 One of the most common isotopes used in radiometric dating is
___________________________ or U-235.
 U-235 is often trapped in the minerals of ____________________ rocks that crystallize
from hot, molten magma.
 As soon as it is trapped in a mineral, U-235 decays into ____________________ or Pb-207.
o What ratio would scientists use to determine how much time has passed since the
mineral was formed?
o Which isotope should there be more of it the rock is older than one half-life?

6. Chapter 16 Lesson 3: Absolute Age Dating – p582–589 – page 6
Draw a graph of U-235 decay.
 One half-life equals .704 billion years
Dating Sedimentary Rock
 How does sedimentary rock form?
 In order to be dated by radiometric means, that ___________________ that formed the
rock must contain U-235.
o The grains of sedimentary rocks come from a variety of weathered rocks from
different _________________________.
 However, by measuring U-235 would scientist be getting the date that the sedimentary
rock formed or the date that the grain of sediment formed?
 Radioactive isotopes within these grains generally record the ages of the
______________________ – not when the sediment was deposited.
 For this reason, sedimentary rock is not as __________________ to date as igneous rock

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More radioactive isotopes used in radiometric dating
 Which has the shortest half-life? _____________________________________________
 Which has the longest? ____________________________________________________
Different Types of Isotopes
 The half-life of U-235 is ____________________________________ million years.
o This makes it useful for dating rocks that are very _______________.
 Many different ______________________ are also used.
o However, would isotopes with short half-lives be useful in dating old rocks?
o Which isotope would be too small to measure, the parent or the daughter?
 Geologists often use a _________________________ of radioactive isotopes to measure
the age of a rock to make it more accurate.
The Age of Earth
 The oldest known rock formation dated by geologists using radiometric means is in
_________________________.
o It is estimated to be between _______________________________ billion years
old.
 However, individual crystals of the mineral zircon in igneous rocks in Australia have been
dated at ________________________ billion years.
 Radiometric dating of rocks from the Moon and meteorites indicate that Earth is
___________________ billion years old.
o Scientists accept this age because evidence suggests that Earth, the ___________,
and ___________________ all formed at about the same time.

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*** Extra Figures***

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