Most of the naturally occurring elements are mixtures of several isotopes. Of the carbon found on this planet, 98.9% is carbon-12, or , which has six protons and six neutrons. (The atomic mass scale is defined so that an atom of carbon- 12 weighs exactly 12 amu.) 1.1% is carbon-13, with one additional neutron. Both of these isotopes are stable, but carbon-14 is radioactive, and is present in minute amounts only because it is being produced constantly by cosmic-ray bombardment of nitrogen in the upper atmosphere. Carbon-14 is the basis of radiocarbon dating. As long as a tree or other organism is alive, it constantly takes in more carbon from its surroundings, and the ratio of to equals that in the atmosphere as a whole. Radioactive decay and replenishment from the atmosphere are in balance. When the tree dies, this intake stops and what little carbon-14 it has begins to disappear. By measuring the ratio of to in a wood or other carbon containing relic from an archaeological site, scientists can calculate how long in the past the specimen ceased to be alive and thus ceased to exchange with its surroundings.