It cannot be used directly to date rocks; however, it can potentially be used to put time constraints on some inorganic material such as diamonds (diamonds could contain carbon-14). Cosmic rays from outer space, which contain high levels of energy, bombard the earth’s upper atmosphere.
The illustration below shows the three isotopes of carbon.
Some isotopes of certain elements are unstable; they can spontaneously change into another kind of atom in a process called “radioactive decay.” Since this process presently happens at a known measured rate, scientists attempt to use it like a “clock” to tell how long ago a rock or fossil formed.
So, a carbon atom might have six neutrons, or seven, or possibly eight—but it would always have six protons.
An “isotope” is any of several different forms of an element, each having different numbers of neutrons.
The atomic number corresponds to the number of protons in an atom.