A Fossil

 Any method of measuring the age of an event or object in years.

 Radioactive decay  Radiometric dating

 Radioactive – unstable isotope Atoms of the same element have the same # of protons but different # of neutrons. Not having the same number of protons and neutrons in an element. Stable- same number of protons and neutrons in an element.

 Tend to break down into stable isotopes of the same or other elements in a process.

Proton (p+) Neutron (n◦) Electron cloud – holds the electrons Nucleus – center of an atom.

 Radioactive decay – when some unstable isotopes decay, a neutron is converted into a proton. In the process, an e- (electron) is released.  ParentDaughter  (6 protons and 8 neutrons) (7 protons and 7 neutrons) Unstable isotope Stable Isotope

 Scientist compare the amount of parent material (p & n) with the amount of daughter material (p & n). The more daughter material there is, the older the rock.

 Based on the ratio of parent material to daughter material.  The ratio is determined by the number (#) of half- life.  (Half – life - the time it takes 1 half-life of radioactive sample to decay). Unstable isotope to stable isotope

 Potassium-Argon Method  Potassium 40 has a half-life of 1.3 billion years.  Decays to Argon and calcium.  Argon is measured as the daughter isotope.  Used to date rocks older than 100,000 years.

 Uranium-Lead Method  Uranium 238 decays to Lead 206.  Half-life of Uranium 238 is 4.5 billion years.  The older the rock, the more daughter material (lead 206) there will be. Younger rock does not have enough daughter material to be measured using this method.  Uranium –Lead can be used to date rocks more than 10 million years old.

 Rubidium-strontium Method –  The unstable parent isotope Rubidium 87 forms the stable daughter Strontium 87.  Half-life of Rubidium 87 is 49 billion years.  Used to date rocks older than 10 million years.

 Carbon 14 method –  Half-life of Carbon 14 is only 5,730 years.  Used mainly for dating things that lived within the last 50,000 years.

 During radioactive decay, an unstable isotope decays at a constant rate and becomes a stable isotope of the same or a different element.  Radiometric dating, based on the ratio of parent to daughter material, is used to determine the absolute age of a sample.  Methods of radiometric dating include potassium-argon, uranium-lead, rubidium- strontium, and carbon-14 dating.