PARADISE VALLEY COMMUNITY COLLEGE
SPRING SEMESTER, 2000
CHEMISTRY 130
SECTION 2230
DR. GERALD A. ROSENTHAL

RADIOCARBON DATING
(before beginning: review radioactivity)

HOW IS CARBON 14 CREATED
    Cosmic radiation (gamma rays) enter the Earth's atmosphere and bombard atmospheric atoms which they break apart to create free neutrons.  These neutrons readily react with 14Nitrogen, the most abundant atom found in our atmosphere.

    147N (the nucleus has 7 protons and 7 neutrons) + neutron
they react together to form
146C (6 protons and 8 neutrons) plus a proton (by-product of the reaction)

In this reaction, stable, nitrogen-14 gains a neutron which would have changed it to a heavier isotope of nitrogen, namely nitrogen-15. However, a proton is lost in the reaction.  Remember, once the number of protons changes, the atomic number changes and a different element is created.  Thus, the final product is carbon-14.  It has the same mass as nitrogen-14 (because of the neutron it gained) but it is a new element (because of the proton it lost).

By this process, carbon-14, A RADIOACTIVE ISOTOPE, is continuously formed in our atmosphere. It reacts immediately with free oxygen in the atmosphere to create radioactive carbon dioxide (14CO2). This radioactive gas is taken in by plants for the purpose of photo-
synthesis, and eventually finds its way into animals that eat the plants.  By these  interactive processes, radioactive carbon-14 finds its way into ALL LIVING SYSTEMS INCLUDING YOUR OWN BODY.  YES, YOU ARE EVER-SO-SLIGHTLY RADIO-
ACTIVE!

HOW IS CARBON-14 DESTROYED
Carbon-14 is radioactive and it  slowly breaks apart or DECAYS.  The decay process involves a spontaneous loss of an electron as a beta particle.

 146C(the nucleus has 6 protons and 8 neutrons)
spontaneously breaks down with the loss of a neutron and the release of an electron
this still leaves an extra proton which is retained to create
147N(7 protons and 7 neutrons) plus a proton (by-product of the reaction)

In this reaction, radioactive, carbon-14 losses a neutron which would have caused the reaction product, to lose mass.  However, at the same time, a  proton was gained  in the reaction. Thus, the final product, nitrogen-14,  has the same mass as carbon-14 (because nitrogen-14 lost a neutron while gaining a proton),  but it is now a new element, nitrogen-14 (because of the proton it gained).  REMEMBER, NEW ELEMENTS ARE CREATED WHENEVER THE NUMBER OF PROTONS CHANGES.

N.B. EACH ELEMENT HAS ITS OWN UNIQUE NUMBER OF PROTONS.  NO TWO ELEMENTS CAN EVER HAVE THE SAME NUMBER OF PROTONS.

GIVEN THIS SOME CAREFUL THOUGHT:

AS LONG AS AN ORGANISM IS ALIVE, IT CAN TAKE IN FRESH CARBON-14 FROM PLANT MATERIAL THAT IT EATS.  WHEN IT DIES, HOWEVER, IT CAN NO LONGER TAKE IN NEW CARBON-14.

FROM THAT POINT ON, THE TOTAL CARBON-14 CONTENT OF THE ORGANISM MUST DECREASE SINCE IT CAN NO LONGER TAKE IN NEW CARBON-14 AND EXISTING CARBON-14 IS BREAKING DOWN.

IF WE KNOW, THE AMOUNT OF CARBON-14 IN THE ORGANISM AT DEATH AND WE KNOW THE RATE AT WHICH CARBON-14 IS BREAKING DOWN (WE KNOW BOTH THINGS), THEN WE HAVE AN EXCELLENT CLOCK, BASED ON THE AMOUNT OF CARBON-14 REMAINING IN THE ORGANISM, TO TELL US HOW LONG AGO THE ORGANISM DIED.

IN CLASS, I SHALL TALK ABOUT HOW WE KNOW BOTH OF THESE IMPORTANT PARAMETERS