Absolute Dating by the Use of 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. Does not mean it isn’t without error Radiometric dating is the most common type of absolute dating.

Atoms and Isotopes: Let’s review the basics Chemical symbol for element The number of protons in an atom determines which element it is. If you change the # of protons, the element changes & the mass changes. If you change the # of neutrons, the element stays the same, but the mass changes ISOTOPES - atoms of the same element that have different numbers of neutrons Mass # (protons + neutrons) Atomic # (protons) What makes each element unique Adding a proton to one element changes it to another, heavier one http://www.saskschools.ca/curr_content/chem30/images/e_deuterium.jpg

Why Are Some Isotopes Radioactive? Isotopes that have the right amount of neutrons are called stable. They always stay the same. Some isotopes have a few too many neutrons or not enough - This makes them unstable and radioactive. The nuclei of these 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 changes, 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. Radioactive atoms decay at a specific rate. Once they decay, they can not change back. 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.

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 If you measure the ratio of parent to daughter isotopes, you can determine how many half-lives have passed. Find the half-life of the parent isotope # of half-lives  length of half-life = age of sample Example: 3 half-lives; 1 half-life = 200 years

Atoms Don’t Age the Way We Do Start with 16 baby aliens 1 Have 70 year half-lives 2 3 4 half-lives = 280 years 4 If living things aged like atoms, the world would be very different. Each atom has a 50% chance of decaying during a half-life. http://www.colorado.edu/physics/2000/isotopes/images/age280_baby.jpg

How to Choose Which Isotope to Use Estimate the age of your sample and choose an isotope with an appropriate range. First find out what minerals are in your sample. The minerals in your rock 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) Ex: Wood, bone, cloth, paper 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 sediment for wales field trip) K-40: feldspar & mica Uranium: zircon

Let’s Practice Absolute Dating