Patricia Hernandez, age 14, of Colton, Calif., for her question:

What causes carbon 14?

You may read that carbon 14 was listed to prove that the posts from an ancient building were cut from trees that grew 5,000 years ago. Carbon 14 may also be used to trace the path of certain chemicals in a living body, to date the age of a mummy or a handful of old campfire ashes.

Carbon is a chemical element needed by living plants and animals to build the firm framework of their cells. Its atoms form glassy diamonds and greasy black graphite and combine with other Elements to form many of the Earth's rocky minerals. Carbon atoms combine with atoms of oxygen to form gaseous carbon dioxide, and specks of solid carbon float in the air as soot and smoke.

The carbon atom contains six protons and six electrons and its atomic number therefore is 6. The atomic number of nitrogen is 7, because its atom has seven protons and seven electrons. In the air, nitrogen atoms are bashed at a steady rate by fast, free flying neutrons from cosmic rays. And at a steady rate a certain number of nitrogen atoms lose a proton and an Electron. The atomic number of these atoms is no. ¬6, and since only carbon can have this atomic number they must be atoms of carbon.

However, such a newly made carbon atom has two more neutrons than ordinary carbon. It is called carbon 14. Carbon 12 and 14 are isotopes of carbon that behave exactly alike and differ only in weight. Plants and animals use both isotopes as if they were the blame.

The heavier isotope, however, tends to be unstable, and carbon 14 is radioactive. Its atoms are formed at a steady fixed rate, and at a steady fixed rate they break apart and decay. The ratio between carbon 12 and carbon 14 in the air remains constant. The rate at which carbon 14 is absorbed by plants and animals remains constant. But when the living cells die, they stop absorbing more chemicals, including carbon 14. The amount of carbon 14 in the tissue begins to break up, and after 5,600 years exactly half of its atoms will decay.

The amount of carbon 14 remaining in an ancient building can be checked. Its ratio to the content of normal carbon can be measured. If this ratio is only half the ratio between carbons 12 and 14 in the air, the old timbers must come from trees that grew more than 5,000 years ago.

Atoms of nitrogen in the air are smashed and become atoms of carbon 14. This radioactive substance has a half life of 5,600 years. In that period of time half the Existing carbon 14 atoms will decay, and after another 5,600 years half of the remaining supply will decay. If no new carbon 14 atoms are added, the ratio between the carbons 12 and 14 in the air would be reduced by half every 5,600 years. But this ratio does not change, because new carbon 14 is being created at a steady rate.