Download Dynamic Earth Unit 2 lesson 3 Absolute Dating

Unit 2 Lesson 3 Absolute Dating
Up and Atom!
What are the parts of an atom?
• Atoms are made up of protons, neutrons, and
electrons.
• Protons are the positively charged particles of
atoms. The relative charge of a single proton is
written as 1+.
• In the unified atomic mass unit (u), the mass
of a proton is about 1 u.
• The number of protons in the nucleus of an atom
is the atomic number of that atom.
What are the parts of an atom?
• Neutrons are particles that have no electrical
charge.
• The mass of a neutron is slightly more than that
of a proton, but it is still close enough to be
considered 1 u.
• Most atoms contain at least as many neutrons as
protons.
Unit 3 Lesson 1 The Atom
How can we describe atoms?
• The atoms of a certain element always have the same
number of protons, but the number of neutrons may
differ.
• Isotopes are atoms of the same element that have
different numbers of neutrons.
• The total number of protons and neutrons in an atom’s
nucleus is its mass number and each isotope has a
different mass number.
• Example: Hydrogen has three natural isotopes: 1H, 2H,
and 3H. Each isotope has a different mass number.
• 1H – 1 proton, 0 neutrons. Mass number = 1
• 2H – 1 proton, 1 neutron. Mass number = 2
• 3H – 1 proton, 2 neutrons. Mass number = 3
What are the parts of an atom?
• Together, protons and neutrons form the nucleus
of the atom.
• The overall charge of the nucleus is equal to the
charge on the total number of protons in the
nucleus.
• The nucleus is small but very dense.
• More than 99% of an atom’s mass is in the
nucleus.
What are the parts of an atom?
• The negatively charged particles of an atom are
called electrons. The charge of a single electron is
represented as 1-.
• Because they move so fast, it is not possible to
determine the exact position and speed of an
electron at the same time.
• So we picture the electrons as being in an electron
cloud around the nucleus.
What are the parts of an atom?
• Compared with protons and neutrons, electrons
have very little mass.
• The number of protons and electrons in an atom
are the same, so their charges are balanced and
the atom has an overall charge of 0.
• An atom can gain or lose electrons to become an
ion, which has a net positive or negative charge.
Absolute Dating
• Determining the actual age of an event or object in years
is called absolute dating.
• Scientists often use radioactive isotopes to find the
absolute age of rocks and other materials.
• Atoms with the same number of protons but different
numbers of neutrons are called isotopes.
• Isotopes are formed from nuclear reactions.
What happens during a nuclear
reaction?
• A nuclear reaction is a change that affects the nucleus of an
atom. It differs from a chemical reaction in several ways.
• One difference is that chemical reactions do not change the
mass of atoms, but nuclear reactions do so by a very small
amount by changing the nucleus of an atom.
• They can change the number of neutrons
• They can change the number of protons changing one type
of atom into another.
(Example: a nuclear reaction can decrease the number of protons
turning a beryllium atom into a lithium atom).
How a nuclear reaction can change an atom into another one.
Nuclear
Reaction
Source
+
+N
+ N
+
N
-
N
-
Beryllium
Lithium
Atom
4 protons
3
4 electrons
3
5 neutrons
N
Because the radiation decreased the number of protons and
electrons from 4 to 3 in the atom, it changed from an atom of
Beryllium to Lithium.
How a nuclear reaction can change an atom into an isotope.
Nuclear
Reaction
Source
N
-
N
-
+
+ +
+
N N
N
Beryllium
Beryllium-8
Atom
Isotope
4 protons
4 electrons
45 neutrons
Because the radiation decreased the number of neutrons from
5 to 4 in the atom, it changed from an atom of Beryllium to the
Beryllium-8 isotope.
What happens during a nuclear
reaction?
• Isotopes of the same element have different
numbers of neutrons and from that, different
mass numbers.
• Carbon atoms have 6 protons and 6 neutrons in
their nucleus, giving them a mass number of 12.
• When carbon atoms lose or gain neutrons, they
become isotopes.
• The mass number is added to the end of the name
of an element to identify isotopes, such as carbon11 and carbon-14.
How can the absolute age of rock be
determined?
• Radioactive isotopes, also called radioisotopes, are
isotopes that are unstable and break down into
other, stable isotopes by a process called
radioactive decay. As they break down, they
release excess energy by emitting radiation in the
form of alpha, beta and gamma rays.
• The radioactive isotope is called the parent
isotope, and the stable isotope formed by its
breakdown is called the daughter isotope.
• Half-life is the time needed for half of a sample
of a radioactive element to undergo radioactive
decay and form daughter isotopes.
• After one half-life has passed, one-half of the
parent isotope has changed into daughter
isotopes.
2865 years = Half-life
Time Elapsed: 5730
= Radiation
= Parent Isotope
= Daughter Isotope
How can the absolute age of rock be
determined?
• Scientists study the amounts of parent and
daughter isotopes to date samples.
• Finding the absolute age of a sample by
determining the relative percentages of a
radioactive parent isotope and a stable daughter
isotope is called radiometric dating.
What is the best rock for radiometric
dating?
• Igneous rocks are the best types of rock samples
to use for radiometric dating.
• When igneous rocks form, minerals in them often
contain only a parent isotope and none of the
daughter isotope.
• This makes the isotope percentages easier to
interpret and helps dating to be more accurate.
Time for a Change
What are some radiometric dating
methods?
• Scientists use many different isotopes for
radiometric dating.
• The type of isotope used depends on the type of
material being dated.
• The half-life of the isotope used is also very
important. It can’t be too short or too long
compared to the age of the sample.
What are some radiometric dating
methods?
• Radiocarbon dating is a method used for dating
wood, bones, shells, and other organic remains.
• All living things have a constant ratio of
radioactive carbon-14 to carbon-12.
• Once a plant or an animal dies, no more carbon is
taken in. The ratio between the isotopes changes
because carbon-14 undergoes radioactive decay.
What are some radiometric dating
methods?
• The half-life of carbon-14 is 5,730 years. The
number of half-lives of carbon-14 that have
passed gives the absolute age.
• Radiocarbon dating can be used to date organic
matter only.
• This method is used to date things that lived in
the last 45,000 years.
What are some radiometric dating
methods?
• Potassium-argon dating is often used to date
igneous volcanic rocks that are 100,000 years to
billions of years old.
• Uranium-lead dating is based on measuring the
amount of the lead-206 daughter isotope in a
sample.
• Uranium-lead dating can be used to determine the
age of igneous rocks that are between 100 million
years and a few billion years old.
Time Will Tell
How is radiometric dating used to
determine the age of Earth?
• Radiometric dating can be used to find the age of
Earth. But there are no Earth rocks which can be
directly studied that are as old as our planet.
• Meteorites are small, rocky bodies that have fallen
from space to Earth’s surface. They are the same
age as the solar system, including Earth.
• The absolute age of meteorites and other rocks in
the solar system is about 4.6 billion years.
Showing Your Age
How can fossils help to determine the
age of sedimentary rock?
• Sedimentary rock layers and the fossils within
them cannot be dated directly.
• But igneous rock layers on either side of a fossil
layer can be dated radiometrically.
• Once the older and younger rock layers are dated,
scientists can assign an absolute age range to the
sedimentary rock layer containing the fossils.
How can fossils help to determine the
age of sedimentary rock?
• Index fossils are fossils used to estimate the
absolute age of the rock layers in which they are
found.
• Once the absolute age of an index fossil is known,
it can be used to determine the age of rock layers
containing the same fossil anywhere on Earth.
How can fossils help to determine the
age of sedimentary rock?
• To be an index fossil, the organism from which it
formed must have lived during a relatively short
geologic time span.
• Index fossils must be relatively common and must
be found over a large area.
• Index fossils must also have features that make
them different from other fossils.
How are index fossils used?
• Index fossils act as markers for the time that the
organisms were alive on Earth.
• Index fossils can also be used to date rocks in
separate areas.
• The appearance of the same index fossil in rock of
different areas shows that the rock layers formed
at about the same time.