Dating with Radioactivity

  • Absolute dating- a method of assigning an age to rocks using radioactive elements.

  • Isotopes- atoms of the same element that have different masses. EX Oxygen-16 and Oxygen-18 are both atoms of oxygen but one has a mass of 16 while the other has a mass of 18.

  • Radioisotopes- are isotopes that have an unstable nucleus. The unstable nucleus will spontaneously rearrange itself to become more stable. When the nucleus rearranges itself it will release energy and may release a particle. The energy released by the nucleus is called radiation. The process of the nucleus rearranging itself is called radioactive decay.

  • Half-life- measures the amount of time it takes for half of a radioisotope to undergo radioactive decay. After one half-life there will be half of the original isotope remaining. For example, thorium-234 is a radioisotope that has a half-life of 24.1 days. If a sample starts with 100 atoms of thorium-234 then after 24.1 days, 50 atoms will remain undecayed. After another 24.1 days 25 atoms remain.

  • Mother- Daughter pairs- The original radioisotope is called the “mother”. Very often when a radioisotope undergoes radioactive decay, it changes into another element called the “Daughter”. For example, carbon-14 is a radioisotope of carbon and when it decays it becomes stable nitrogen-14.

  • Radiometric Dating- a method for determining the absolute ago of a rock sample by measuring the mother:daughter ratio in the sample and knowing the half-life so that the amount of time since the rock first formed can be calculated. This method only works on igneous rocks. However, fossils only exists in sedimentary rock. Therefore, the absolute age range of a fossil is determined by combining relative dating techniques with the absolute dating of igneous rock.

  • Radiocarbon Dating- living things contain a lot of carbon in their tissues. While an organism lives the ratio of radioisotope carbon-14 to stable carbon-12 is constant. However once an organism dies, the amount of carbon-14 in its tissue steadily declines by radioactive decay. Therefore, the amount of carbon-14 found in remains of once living tissue can be used to calculate how long ago the organism died.

  • Limitations of radiometric dating

  1. Can only date the igneous rock

  2. rock cannot have been altered by weathering or metamorphasis.

  3. The age of the sample cannot be olderr than 10 half-lives because not enough of the original radioisotope remains to be accurately measuered.