Transcript Radiometric Dating - EHS

Finding Absolute Ages Using
Radioactive Isotopes
What is Absolute Dating?
Age of fossil or rock is given in years
instead of relative terms like before and
after, early and late.
Error is quantitative and measureable
Radiometric dating is the most common
type of absolute dating.
Atoms and the Periodic Table
What are atoms made of?
Nucleus = mass of the atom
 Protons + charged (p+)
 Neutrons no charge (no) made of both p+ and e Atomic Mass = #no + #p+
Orbitals = volume of the atom
 Electrons – charge (e-)
 Atoms are neutrally charged:
#e- = #p+
Elements
 A pure chemical substance
consisting of a single type of atom
 Divided into metals, metalloids, and
nonmetals.
 Distinguished by the atomic number
= the number of protons
 Change the # of p+, change the mass
AND the type of element
 Change the # of no, change the mass
only  creates an ISOTOPE
Chemical symbol
for element
Mass #
(protons +
neutrons)
A
Z
X
Atomic # (protons)
Isotopes
 A variation of an element’s
atoms
 Same number of protons
 Different number of neutrons
Different atomic mass
 ISOTOPES - atoms of the
same element that have
different numbers of neutrons
Radioactive Decay
 The process by which a
nucleus of an unstable atom
loses energy by emitting
radiation
 The atom spontaneously
changes into an
atomic nucleus of either
 a different element, OR
 the same element with a
different MASS
 There are 3 types of radioactive
decay are:
•
•
•
Alpha radiation can be stopped by PAPER.
Beta radiation can be stopped by WOOD.
Gamma radiation can be stopped by LEAD.
Alpha Decay α
 Loss of an alpha particle
 2 p+ & 2 no (alpha particle) are emitted from the
nucleus.
 atomic number of the element decreases by two
because 2 p+ are lost and the atomic # is determined
by the # of p+
 the atomic mass is decreased by four because each p+
and no has an atomic mass of one and there are a total
of four in an alpha particle
Beta Decay β
 a neutron decays emits an electron (beta particle)
and becomes a proton.


atomic number is increased by one because the neutron
releases an electron and leaves a proton behind
the atomic mass does not change (because one neutron
was lost and one proton was gained so they cancel each
other out regarding the atomic mass)
Electron Capture Decay, γ
 a proton captures an electron and converts to a
neutron.
 Loss of a Gamma Ray
 no change in the atomic mass because a
gamma ray is a burst of energy without mass
 Atomic Number decreases by 1
Why Are Some Isotopes Radioactive?
 Stable Isotopes have a constant
number of neutrons and do not
spontaneously change
 Radioactive Isotopes isotopes
have too few or too many
neutrons making them unstable.
 The nuclei of radioactive atoms
change or decay by giving off
radiation in the form of particles or
electromagnetic waves until the
atom reaches a stable state.
Radioactive Decay



During radioactive decay, the number of protons in the
atom can change and one element transforms into
another.
Parent isotopes decay into daughter isotopes.
Radioactive Decay is like popping popcorn.
 Each radioactive parent always
decays to a specific daughter.
 There is no way to predict which
atoms will decay first.
 Once they decay, they cannot
change back.
 Radioactive atoms decay at a
specific rate = HALF-LIFE
How Long Does Radioactive Decay Take?
 Half-Life - the time it takes for half of the radioactive or
parent isotopes in a sample to decay to daughter
isotopes.
 Each parent has a 50% chance of decaying during 1 half-life
 Measured in seconds, minutes, years, etc.
 Each isotope has its own unique half-life.
 From thousandths of a second to billions of years
Starting the Stopwatch
HOW TO FIND A RADIOMETRIC AGE:
 Measure the ratio of parent to daughter isotopes
 Look up the half-life of the parent isotope (determined experimentally)
 # of half-lives  length of half-life = age of sample
 Example: 3 half-lives; 1 half-life = 200 years
 3 x 200 = 600 years old
How to Choose Which Isotope to Use
 Use Relative Dating to estimate
the age of your sample and
choose an isotope with an
appropriate range.
 Determine the minerals in the
sample.
Feldspars & Micas: use K-Ar
Zirons: use U-Pb
Bone or Wood: use C-N
 The minerals need to have the
element you want to use for
dating.
 Carbon-14 can only be used to
date samples that were once
living (organic) like wood, bone,
cloth, paper, etc.
Let’s Practice Absolute Dating
 Nickel-63 (parent) decays to Copper-63 (daughter)
 Half-Life = 100 years
 Find the ages of the following samples
Mass of
Parent
Mass of
Daughter
50 g
50 g
25 g
75 g
12.5 g
87.5 g
6.25 g
93.75 g
Ratio of
Parent to
Daughter
Number of
Half-Lives
Age of
Sample
Mass of
Parent (Ni63)
Mass of
Daughter
(Cu-63)
Ratio of
Parent to
Daughter
Number of
Half-Lives
Age of
Sample
50 g
50 g
1:1
1
100 years
25 g
75 g
1:3
2
200 years
12.5 g
87.5 g
1:7
3
300 years
6.25 g
93.75 g
1:15
4
400 years
 But what if the data is not so “nice”?
 What would the age be of a sample with 30g of Ni-63 and 70g of
Cu-63?
 What could you create to make this problem easier to solve?