2Basic Atomic Structure Each atom has a nucleus containing protons and neutrons and that nucleus is orbited by electronsElectrons have a negative electrical charge and protons have a positive chargeNeutrons have no chargeThe atomic number of an element is the number of protons in the nucleus, atoms of the same element always have the same atomic numberAn atom’s mass number is the number of protons and neutrons in a nucleusThe number of neutrons can vary in a nucleus, and these variants, or isotopes, have different mass numbers
3RadioactivityThe forces that bind protons and neutrons together in a nucleus are usually strongWhen nuclei are unstable, they spontaneously break apart, or decay, in a process called radioactivityRadioactivity – the spontaneous decay of certain unstable atomic nucleiAn unstable radioactive isotope of an element is called the parentThe isotopes that result form the decay of the parent are called the daughter productsRadioactive decay continues until a stable or non-radioactive isotope is formed
5Half-LifeHalf-Life – the time for one half of the atoms of a radioactive substance to decay to its stable isotopeIf the half-life of a radioactive isotope is known and the parent/daughter ratio can be measured, the age of the sample can be calculated
7Concept Check What is a half-life? The amount of time necessary for one half of the nuclei in a sample to decay to its stable isotope.
8Radiometric DatingRadiometric Dating – the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopesEach radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occursThe products of decay have also been accumulating at a constant rateAs uranium decays, atoms of the daughter product are formed, and measurable amounts of lead eventually accumulateAn accurate radiometric date can be obtained only if the mineral remained in a closed system during the period since its formationAlthough the basic principle of radiometric dating is simple, the actual procedure is quite complex
9Radioactive Isotopes Frequently Used in Radiometric Dating Radioactive ParentStable Daughter ProductCurrently Accepted Half-Life ValuesUranium-238Lead-2064.5 Billion YearsUranium-235Lead-207713 Million YearsUranium-232Lead-20814.1 Billion YearsRubidium-87Strontium-8747.0 Billion YearsPotassium-40Argon-401.3 Billion Years
10Concept Check Why is a closed system necessary in radiometric dating? An accurate radiometric date can be obtained only if the mineral remained in a closed system since its formation.
11Dating with Carbon-14Radiocarbon (Carbon-14) Dating – method for determining age by comparing the amount of carbon-14 to the amount of carbon-12Carbon-14 is continuously produced in the upper atmosphereIt becomes incorporated with carbon dioxide, which is absorbed by living matterAll organisms—including you—contain a small amount of carbon-14; when an organism dies, the amount of carbon-14 gradually decaysBy comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determinedBecause the half-life of carbon-14 is only 5730 years, it can be used to date recent geologic events up to about 75,000 years agoCarbon-14 is a valuable tool to anthropologists, archeologists, and historians
13Concept Check What is compared when dating with carbon-14? The ration of carbon-14 to carbon-12.
14Importance of Radiometric Dating Radiometric dating has produced dates for thousands of geologic events in Earth’s historyRocks formed on Earth have been dated to be as much as 4 billion years oldMeteorites have been dated at 4.6 billion years oldRadiometric dating has supported the ideas of James Hutton, Charles Darwin, and many others who inferred the geologic time must be immense