1. Finding Absolute Ages Using Radioactive Isotopes

2. 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.

3. 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+

4. 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)
Atomic # (protons)

5. 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
What makes each element unique
Adding a proton to one element changes it to another, heavier one

6. 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:

7. Alpha radiation can be stopped by PAPER.
Beta radiation can be stopped by WOOD.
Gamma radiation can be stopped by LEAD.

8. 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

9. 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)

10. 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

11. 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.

12. 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
Fast popping at beginning then slows down at end
Atomic nuclei are held together by an attraction between the protons and neutrons (called the nuclear force), which has to be greater than the electrostatic repulsion between the protons within the nucleus in order for the nucleus to remain stable. In general, the number of neutrons in an atomic nucleus must at least equal the number of protons because electrostatic repulsion prohibits denser packing of protons. If there are too many neutrons, the nucleus has the potential to become unstable. Decay happens spontaneously at any time when the electrostatic repulsion is greater than the nuclear force that holds the nucleus together.

13. 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

14. 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

15. 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.
At death, organisms stops taking in C-14 in food and air and C-14 decays to N-14
Add in pictures of the minerals above and of artifacts that can be dated with C-14 (example of dating wood to date)
Feldspars & Micas: use K-Ar
Zirons: use U-Pb
Bone or Wood: use C-N

16. 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
25 g
75 g
12.5 g
87.5 g
6.25 g
93.75 g

17. Ratio of Parent to Daughter
Mass of Parent (Ni-63)
Mass of Daughter (Cu-63)
Ratio of Parent to Daughter
Number of Half-Lives
Age of Sample
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?