3.57. Radioisotopes http://www.ck12.org
carbon-12, where 12 is the number of protons (6) plus neutrons (6). This gives carbon-12 nuclei a 1:1 ratio of protons
to neutrons, so carbon-12 nuclei are stable.
Some carbon atoms have more than 6 neutrons, either 7 or 8. Carbon atoms with 8 neutrons are called carbon-14 (6
protons + 8 neutrons). The nuclei of carbon-14 atoms are unstable because they have too many neutrons relative to
protons, so they gradually decay.
Q:What is the proton-to-neutron ratio of carbon-14 nuclei?
A:With six protons and 8 neutrons, the ratio is 6:8, or 1:1.3.
Q: How is carbon-14 used to estimate the ages of fossils? A: Living things take in carbon, including tiny amounts of
carbon-14, throughout life. The carbon-14 constantly decays, but more carbon-14 is taken in all the time to replace
it. After living things die, no new carbon-14 is taken in, and the carbon-14 they already have keeps decaying. The
older a fossil is, the less carbon-14 it still has, so the remaining amount can be measured to estimate the fossil’s age.
You can learn more about carbon-14 dating at this URL: http://www.youtube.com/watch?v=31-P9pcPStg.
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/82380FIGURE 3.106
Periodic Table of the ElementsRadioactive Elements
In elements with more than 83 protons, all of the isotopes are radioactive. In theFigure3.106, these are the elements
with a yellow background. The force of repulsion among all those protons makes the nuclei unstable. Elements with
more than 92 protons have such unstable nuclei that they don’t even exist in nature. They have only been created in
labs.