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

RADIOCARBON DATING
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.

Refresh your memory about our discussion of the three isotopes of hydrogen.
Carbon also has several isotopes.  Most of the carbon of our Universe occurs as carbon-12 but there is also a little bit of radioactive carbon-14 as well.

IT IS A SIMPLE MATTER TO DETERMINE THE EXACT AMOUNT OF CARBON IN A GIVEN BIOLOGICAL SAMPLE.

WE KNOW VERY ACCURATELY HOW MUCH OF THE TOTAL CARBON IS MADE UP OF EACH OF CARBON'S ISOTOPES INCLUDING ITS RADIOACTIVE, CARBON-14 ISOTOPE. Therefore, once we know the total amount of carbon in the sample, we know exactly how much radioactive carbon-14 was initially present in the sample.

AS THE SAMPLE AGES, THE AMOUNT OF CARBON-12 NEVER CHANGES BECAUSE IT IS A STABLE ISOTOPE.  FOR EXAMPLE, IF WE ARE TRYING TO DATE A PIECE OF BONE, THE AMOUNT OF CARBON-12 IN THE BONE NEVER CHANGES.  ON THE OTHER HAND, THE AMOUNT OF CARBON-14 IS CONSTANTLY CHANGING BECAUSE IT IS A RADIOACTIVE ISOTOPE.

IF WE KNEW THE RATE OF CHANGE, THAT IS THE DECOMPOSITION RATE OF CARBON-14, THEN ALL WE WOULD HAVE TO DO IS TO COMPARE THE ORIGINAL AMOUNT OF CARBON-14 THAT WAS IN THE BONE WHEN THE ANIMAL DIED TO THE AMOUNT OF RADIOACTIVE ISOTOPE REMAINING IN THE BONE.

THIS TOO WE CAN DO BECAUSE WE KNOW THE EXACT RATE OF DECOMPOSITION OR DECAY OF CARBON-14.

HALF LIFE (T1/2)

.  AND THEREFORE TO ALSO KNOW HOW MUCH CARBON-14 WAS IN THE SAMPLE WHEN THE SUBJECT OF THE SAMPLE DIED.