Absolute Dating with Radioactivity

What is Radioactivity? When the forces binding protons and neutrons together are not strong enough, and the nuclei spontaneously break apart (decay) Isotopes—elements with the same number of protons, but different numbers of neutrons

Common Types of Radioactive Decay Alpha Particles— α particles –Composed of 2 protons and 2 neutrons –Emitted from the nucleus –Mass number of the isotope is reduced by four –The atomic number is decreased by two

Common Types of Radioactive Decay Beta Particles— β particles –Composed of one electron –Comes from a neutron –Increases protons by one –Atomic number increases by one

Common Types of Radioactive Decay Electron Capture –An electron is captured by the nucleus –It combines with a proton and forms a neutron –Mass number remains unchanged –One less proton; atomic number decreases by one

Half-Life The time required for one-half of the nuclei in a sample to decay A common way of expressing the rate of radioactive disintegration

Radiometric Dating Radioactivity (unstable nuclei) is a reliable means of calculating the ages of rocks and minerals The rates of decay for many isotopes have been precisely measured A correct date is not possible unless the mineral remained a closed system during the entire period since its formation

Carbon-14 Half-life is 5730 years old Used to date recent events in history While an organism is alive, the proportion of carbon-14 to carbon-12 is constant When the organism dies, carbon-14 decays to nitrogen-14 by beta emission Scientists compare the proportions of carbon-14 and carbon-12

Importance of Radiometric Dating Have produced thousands of dates for events in Earth’s history Some rocks have been dated at more than three billion years Proven that geologic time is immense

Difficulties in Dating the Geologic Time Scale Not all rocks can be dated radiometrically Most sedimentary rock cannot be accurately dated because the particles that formed it are much older than the rock in which they occur

Difficulties in Dating the Geologic Time Scale Metamorphic rock is difficult to date because the age of a particular mineral in the rock is not necessarily the same as when the rock initially formed Use cross-cutting relationships