How accurate is the relative time scale?
What methods can be used to determine how accurate this scale is?
Radiometric dating
Dendrochronology.
What is Radiometric dating?
Uses radioactive decay of certain elements to calculate ages
How does this work?
First a quick review of atomic structure
Review:
What is an element?
What is an atom?
Atoms are the smallest unit that retain all the basic properties of an element
Atoms are made up of three basic particles
What three primary particles are found in an atom?
Protons, Neutrons, Electrons
Protons
Have a charge of +1 and have a mass of 1amu
are found in the nucleus
Electrons
have a charge of –1 and have no measurable mass
are found in the shells or orbitals (same thing as energy levels)
Electrons are VERY mobile and move easily
Neutrons
have no charge (are neutral) and have a mass of 1amu
are found in the nucleus
can be created by joining a proton & electron
Basic rules for atoms:
1) atoms are neutral
Equal numbers of electrons and protons
2) How do we calculate the atomic mass of an atom? (A)--shorthand notation
Mass of an atom = # protons + # neutrons
3) What is the atomic number of an element? (Z)--shorthand notation
Atomic number is the same as the # of protons
Electrons—quick overview
Important for bonding atoms together (more in a week or so)
Bonding occurs due to valence electrons
Electrons in the outer shell or orbital
Periodic Table
Shows all the known elements
symbol, atomic # (Z), and atomic weight (A)
Broken into two basic groups
Metals: conductor, most solid at room temp, ductile, malleable, shiny. Loses
electrons easily
Non metals: most are gas or liquid at room temp accepts electrons easily.
Each group of elements has similar properties
Families have same number of valence electrons
Columns 1,2, 13-18
transition metals (3-12) vary in number of valence electrons
Every known element has several different isotopes.
Isotope: an element that varies the number of neutrons in its structure causing its
weight to change
Not all isotopes are stable
Ex// Carbonà Stable isotopes: 13C and 12C Unstable: 14C
In this case stability refers to the nucleus of the atom (more in a minute)
Strong Force
If the atomic wt. of an atom is 63.546 and the atomic number is 29, how many neutrons are there? Why isn’t it a whole number?
Physicists believe that when protons and neutrons come together in the nucleus, some
of their mass is converted to energy. The energy binds them together—Strong Force
Most atoms on earth have a stable nucleus (99.9%)à the nucleus doesn’t ever change
Some elements are not stable thoughà what happens to them?
Over time spontaneously changes into another element.
This spontaneous decay emits energyàRadiation
Process is known as radioactive decay or radioactivity.
How does spontaneous decay occur?
Alpha decay: Nucleus emits an alpha particle
alpha particle = 2 protons and 2 neutrons
new element has atomic mass decrease (4)
atomic number decrease (2)
Beta decay: Nucleus emits a beta particle
Neutron decays à proton and electron
beta particle is the electron
no change in mass (exchange 1 N for 1 P)
increase atomic # by 1
Electron Capture
electron is captured by the nucleus
joins with a proton àneutron is created
atomic mass is the same; atomic number decreases by 1
TERMS TO KNOW:
Parent or Mother element:
element before decay
Daughter:
new element create because of decay
When an element decays, not all the atoms decay at the same time
1/2 life: The amount of time it takes for ½ of the parent to decay into the
daughter.
ex//
Start with 100 atoms of x. 20 minutes later there are only 50 of x but also 50 of y. Which is the parent? Which is the daughter? The ½ life?
you now have 50 of x and 50 of yà 20 minutes pass. How much of each do you have? 25 of x and 75 of y. another 20 minutes passà 12.5 of x and 87.5 of y.
for each of these times, what is the ratio of parent to daughter?
½ lives can vary from fraction of seconds for some elements (Uranium 227) to billions of years (Uranium 238).
If the ½ life is billions of years how do we know?
Radioactive decay is used to calculate the age of rock units
must know what the parent and daughter elements are
must know the 1/2 life of the parent
measure the amount of each element
calculate the age based on quantity, ratio of elements and 1/2 life
Limitations of method?
Requires a closed system
Use the correct parent/daughter pair
Type of rock
Closed system
system can not allow any parent/daughter to be added/removed
if parent is added what does this do to the calculated age of the rock
if daughter is added what does this do to the calculated age of the rock
if daughter is removed what does this do to the calculated age of the rock?
Use the correct Pair
Parent Daughter 1/2-life
Uranium-238 Lead-206 4.5 bil. Years
Rubidium-87 Strontium-87 47 bil. Years
Potassium-40 Argon-40 1.3 bil. Years
Carbon-14 Nitrogen-14 5700 years
If rocks are 1 million years old what is the problem of dating using the Rubidium/strontium pair?
Can the carbon/nitrogen pair be used to directly age date rocks?
Why or why not?
Radiometric dating techniques reveal the last crystallization of the rock
Which types of rocks can this be used for?
Igneous? Sedimentary? Metamorphic?
How are undatable rocks dated?
Commonly used radioactive elements
oxygen and hydrogen--> water and air
potassium, rubidium, uranium--> crustal rocks
carbon--> organic matter
Carbon 14 dating
carbon has 3 isotopes
12C, 13C, 14C
12C and 13C are stable; 14C is not stable
14C is created in the atmosphere
12C and 14C and taken in by organic means
when organism dies 12C is stable 14C is not--decays
compare quantity of 12C to 14C to calculate age
1/2 life of 14C--> 14N is ~5700 years
allows age dating of organisms ~70,000 years old
why can’t you date older organisms than this?