Radiometric Dating

  Radiometric Dating

The aging process in human beings is easy to see. As we age, our hair turns gray, our skin wrinkles etc

However, rocks and other objects in nature do not give off such obvious clues about how long they have been around. So we rely on radiometric dating to calculate their ages.

Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. Different methods of radiometric dating can be used to estimate the age of a variety of natural and even man-made materials.

The whole process is only possible because of radiactive decay.

  How radioactive dating works?

Example - C 14 Dating

There are two basic forms of carbon: one that occurs naturally, called carbon-12 (12C), and one that forms from processes acting on nitrogen in the atmosphere, called carbon-14 (14C).

When a cow eats grass, its body absorbs the carbon (both12C and 14C) in the plant. When the cow dies, it stops taking in carbon (for obvious reasons). The amount of 12C in the cow’s body stays the same after death, but the amount of 14C changes because it returns to nitrogen.

As time goes on, the amount of 14C continues to decrease until nothing is left

When a paleontologist finds a bone (or a piece of wood), measure the amount of 14C and 12C it contains. Based on how much 14C is left, can supposedly calculate when the animal (or plant) died.

Elements Used in dating

Geologists regularly use five parent isotopes to date rocks:

These parent radioisotopes change into daughter lead-206, lead-207, argon-40, strontium-87, and neodymium-143 isotopes, respectively. Thus geologists refer to

  • uranium-lead (two versions),
  • potassium-argon,
  • rubidium-strontium,
  • or samarium-neodymium dates for rocks.

Note that the carbon-14 (or radiocarbon) method is not used to date rocks because most rocks do not contain carbon.


Radioactive  dating  is a way of determining the age of sample of material  using  the decay rates of radioactive  elements to provide  a clock.

Measuring the ratio of parent to daughter isotopes, determines how many half-lives have passed

——Number of half-lives X length of half-life = age of sample

  Atoms and Isotopes

—The number of protons in an atom determines which element it is. Change in number of protons, changes the element & its mass changes. However, change in the number of neutrons, the element stays the same, but the mass changes.

——Isotopes - atoms of the same element that have different numbers of neutrons


Why Are Some Isotopes Radioactive?

Isotopes that have the right amount of neutrons are called stable.  They always stay the same.

Some isotopes have a few too many neutrons or not enough - This makes them unstable and radioactive.

The nuclei of these radioactive atoms change or decay by giving off radiation in the form of particles or electromagnetic waves until the atom reaches a stable state.


  Types of Radioactive decay

Alpha decay - Alpha decay is caused when there are too many protons in a nucleus. In this case the element will emit radiation in the form of positively charged particles called alpha particles.

Beta decay - Beta decay is caused when there are too many neutrons in a nucleus. In this case the element will emit radiation in the form of negatively charged particles called beta particles.

Gamma decay - Gamma decay occurs when there is too much energy in the nucleus. In this case gamma particles with no overall charge are emitted from the element.