How do scientists know for certain, how old the Earth is?
We must first understand something about atoms. Every atom has a small nucleus surrounded by a cloud of particles called electrons (with a negative electrical charge). In the nucleus there are protons (with a positive charge) and neutrons (with no electrical charge). Some atoms are unstable and decay by emitting a certain kind of particle. As they decay, they change from being an atom of one element into an atom either of another element or of the same element with a lighter nucleus. An element that exists in more than one form is called an isotope, and an unstable isotope is called a radioisotope. The time taken for one half the atoms of a radioactive substance to decay into its daughter element is called its half-life. In simple terms, radioisotope dating works by measuring the ratio of parent to daughter in a rock and calculating how long it took for the one to decay into the other. It can be applied only to rocks that formed directly out of magma (igneous rocks). The age of sedimen
