G1-7 Absolute Age: Radioactive decay
Frederick Soddy (1877–1956), an English chemist, won the Nobel Prize in Chemistry in 1921 for his discovery of isotopes.

Absolute age refers to the actual age in years of an object.

The most common method used for determining the absolute age of rocks is based on the process of radioactive decay. Radioactive decay is the process whereby unstable atoms in an element break apart and form new atoms. (See “A2: Where does the Heat Come From?” for how radioactivity acts as an internal heat source for the Earth.)

In this example of radioactive decay, uranium-238 (U-238), the original parent element, breaks down into a series of unstable daughter elements before finally arriving at the final daughter element, lead-206 (Pb-206), which is stable.

Atoms consist of a nucleus which is surrounded by a cloud of negatively charged electrons. The nucleus contains two kinds of particles: protons which have a positive charge, and neutrons which have no electric charge. The number of protons in an atom, known as the atomic number, is what defines an element. The Periodic Table is set up by atomic number. Although the number of protons that defines an element is fixed, the number of neutrons in that element may vary. Atoms of an element that have different numbers of neutrons are known as isotopes.

Carbon, for example, has numerous isotopes. carbon-12 (C-12) has 6 neutrons (and 6 protons) and is stable, while carbon-14 (C-14) has 8 neutrons (and 6 protons) and is unstable. carbon-14 will break apart over time, and decay into a new element, nitrogen-14 (N-14). The original element in this process, carbon-14, is known as the parent element. The product of the radioactive decay, nitrogen-14, is known as the daughter element.

Did You Know?
A Glowing Discovery In 1896, French physicist Henry Becquerel (1852–1908) noticed that pitchblende (uranium) would cause photographic plates to become exposed. This discovery of radioactivity resulted in Henri Becquerel being named as one of the winners of the Nobel Prize for Physics in 1903. The unit of radioactivity, the becquerel, is named in his honor.
Word Link
Isotope: This word was coined in 1913 by English chemist Frederick Soddy, who discovered isotopes. He derived the word isotope by combining the Greek word isos which means equal or the same with the Greek word topos meaning place. Isotope literally means having the same place (on the Periodic Table).