Finding 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 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+ 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) A Z X Atomic # (protons) 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 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: • • • Alpha radiation can be stopped by PAPER. Beta radiation can be stopped by WOOD. Gamma radiation can be stopped by LEAD. 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 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 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 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 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 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. Feldspars & Micas: use K-Ar Zirons: use U-Pb Bone or Wood: use C-N 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. 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 50 g 50 g 25 g 75 g 12.5 g 87.5 g 6.25 g 93.75 g Ratio of Parent to Daughter Number of Half-Lives Age of Sample Mass of Parent (Ni63) 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?