Radioactive Decay, Half-Life, and Carbon Dating Help

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Absolute Time

The second method to study geological time is done by the chemical and radiological testing of different isotopes (forms of the same element) within rock and mineral samples. This is called absolute or chronometric time measurements.

By using rock and fossil samples that have been classified as to their relationship to one another, laboratory testing can then determine a sample’s age and time placement. The two methods work together to give scientists an accurate time picture of a sample’s age.


The radioactive properties of different elements were discovered in 1896, by Antoine Becquerel when he discovered that a photographic plate in his lab, never exposed to sunlight, had somehow become exposed. The only possible culprit was a nearby uranium salt sitting on the laboratory bench top.

The term, radioactivity , was first used by French scientist, Marie Curie, in 1898. Marie Curie and her physicist husband, Pierre, found that radioactive particles were emitted as either electrically negative (–) called beta (β) particles, or positive (+) called alpha (α) charged particles.

Radioactivity is the characteristic of an element to change into another element through the loss of charged particles from its nuclei.

Following the further understanding and discovery of radioactive breakdown products, researchers began to see a use for radioactivity and radioactive elements, in the study of rock, mineral, and fossil samples.

Nuclear Reactions

Most chemical reactions are focused on the outer electrons of an element, sharing, swapping, and bumping electrons into and out of the joining elements of a reaction. Nuclear reactions are different. They take place within the nucleus.

There are two types of nuclear reactions. The first is the radioactive decay of bonds within the nucleus that emit radiation when broken. The second is the “billiard ball” type of reactions, where the nucleus or a nuclear particle (like a proton) is slammed into by another nucleus or nuclear particle.

Radioactive Decay

A radioactive element, like everything else in life, decays or ages. When uranium decays over billions of years, it goes through a process of degrading into lower and lower energy forms until it settles into one that is stable.

The ages of the most ancient rocks can be found by measuring the decay of specific isotopes that are not stable, but break down to other element forms. The sample is dated using testing techniques known as radiometric dating . This considers all the various melting and environmental influences that have affected the sample.

When a radioactive element decays, different nuclear particles are given off. These speeding radiation particles can be separated by an electric (magnetic field) and detected in the laboratory:

beta (β) particles = negatively (–) charged particles alpha (α) particles = positively (+) charged particles gamma (γ) particles = electromagnetic radiation with no overall charge (similar to x-rays)

The age of geological samples is found by measuring radioisotope decay . Decay of radioactive isotopes is affected by the stability of an element at a certain energy level (where its electrons are stable and bonded). Bismuth (Bi) is the heaviest element in the Periodic Table with a minimum of one stable isotope. All other heavier elements are radioactive.

Geologists study isotopes of different chemical elements to find the rate of decay over time. Depending on the rate of decay of a sample, an estimate of its age can be done. It is also possible to find and compare the radioactive decay of a sample to the rock in which it was found. This gives geologists another clue as to the life cycle and history of the specimen, as well as a hint as to how it was deposited.

Isotopes are chemically identical atoms of the same element that have different numbers of neutrons and mass numbers.

A rock sample’s age can be found by comparing three pieces of information:

1. the amount of the original element or parent isotope ,

2. the amount of the new element or daughter isotope formed, and

3. the rate of decay of a specific radioactive isotope present in the rock.

A mass spectrometer is an instrument that measures the ratios of isotopes in samples. Uranium has all radioactive isotopes while potassium has only one. By noting the rate of decay that uranium-238 displays while losing electrons and alpha particles and trying to become the more stable lead-206, scientists then test other radioactive samples with a similar rate of decay to the stable lead form. Meteorites have been found to be as old as the Earth and older by using this method.

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