An element can exist in different forms which have same chemical properties but different atomic weights. They have same atomic mass because number of protons are identical but additional neutrons make then different in weight. Such atoms are called isotopes.
For example carbon which has 6 protons and 6 neutrons have an isotope which have 6 protons but 8 neutrons. They are represented as C14 and C12. Addition of extra neutrons make them less stable and many of them are radioactive.
The rate at which a particular atom decays is independent of temperature, light or darkness. During decay the atom is trying to achieve state of minimum energy and produces atoms with lower atomic numbers and energy.
The rate of decay is expressed in terms of half life. For example a given weight of C14 atoms will become half in weight in 5600 years precisely. Then the remaining mass will again take 5600 years to become 1/4 th and so on.
This property has many uses and one of the most popular one is carbon dating to determine the age of wood.
Vegetation absorbs CO2 and the same C14/C12 proportion is found in wood as in the atmosphere as long as the tree is alive. But, after a tree is felled it no longer absorbs CO2 from the atmosphere. Inside the dead wood, the C14 brothers slowly disappear, and are not replaced.
So, a piece of wood from a Pharaoh’s tomb contains a far lower C14/C12 proportion than a tree that has recently been felled.
This type of clock is particularly useful to archaeologists, and allows them to date wood between 200 and 20,000 years old. The ratio of C14/C12 is measured using a ratio recording mass spectrometer.