finding absolute ages using radioactive isotopes
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Finding Absolute Ages Using Radioactive IsotopesFinding 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 atomProtons + charged (p+)Neutrons no charge (no) made of both p+ and e-Atomic Mass = #no + #p+
Orbitals = volume of the atomElectrons – charge (e-)Atoms are neutrally charged:
#e- = #p+
ElementsA pure chemical substance
consisting of a single type of atomDivided 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
Atomic # (protons)
Mass # (protons + neutrons)
Isotopes
A variation of an element’s atoms Same number of protons Different number of neutronsDifferent atomic mass
ISOTOPES - atoms of the same element that have different numbers of neutrons
Radioactive DecayThe 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
MASSThere 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 αLoss of an alpha particle2 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
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
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.
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
HOW TO FIND A RADIOMETRIC AGE:Measure the ratio of parent to daughter isotopesLook 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
Starting the Stopwatch
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. 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.
Feldspars & Micas: use K-Ar
Zirons: use U-Pb
Bone or Wood: use C-N
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
Ratio of Parent to Daughter
Number of Half-Lives
Age of Sample
50 g 50 g
25 g 75 g
12.5 g 87.5 g
6.25 g 93.75 g
Mass of Parent (Ni-
63)
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?