40 THE CHEMISTRY OF LIFE 



isotopes we do not give them a separate name as is done for the isotopes 

 of hydrogen. 



Some isotopes of atoms are radioactive; that is, they give off high 

 energy rays which can be detected by means of special instruments 

 such as the Geiger counter. Such isotopes are sometimes called tagged 

 atoms and are very valuable in biological research. The fate of cer- 

 tain minerals in an animal or plant can be detected by these tagged 

 atoms. A small amount of a radioactive isotope of iodine can be given 

 a guinea pig in its food and a day or two later the animal can be dis- 

 sected and its organs analyzed with a Geiger counter. This instrument 

 will show that most of the iodine has been taken up by the thyroid 

 gland. Likewise it is possible to trace minerals from the soil and find 

 out which plant tissues utilize them to the greatest degree. 



Carbon fourteen (C 14 ) is a radioactive isotope of carbon (C 12 ) and 

 is of great value in establishing the age of the remains of ancient forms 

 of life. Since a certain proportion of the carbon in the carbon dioxide 

 of the air is this radioactive form, all living things have this same 

 proportion of it in their bodies. At death no more carbon fourteen is 

 added and that which is present is gradually converted into ordinary 

 carbon through the loss of electrons in the course of the continual 

 radiation. It takes about 4,700 years for the amount of radioactive 

 carbon to be reduced to one half, another equal period of time for it to 

 be reduced one half again, and so on. Now, if an animal is uncovered 

 and the remains are found to contain one eighth the amount of radio- 

 active carbon that is found in living animals, we may conclude that it 

 died approximately 37,600 years ago. There are some qualifications 

 which must be considered in this method of reckoning time, but it is 

 generally considered to be extremely accurate. 



Ionization of Molecules 



Whenever a teaspoon of sugar is put into water and stirred, we 

 say that it goes into solution. The water looks the same as before, but 

 it is somewhat thicker in consistency and it tastes sweet. What hap- 

 pened to the sugar? It has broken down into sugar molecules which 

 become evenly dispersed throughout the water. If we dissolve table 

 salt in water, however, the salt breaks down into particles below the 

 molecule size which are called ions. A molecule of table salt is com- 

 posed of two atoms, sodium (Na) and chlorine (CI). Its chemical 

 formula is NaCl. Now if this salt breaks down below the molecule 

 size, you might think that it would break down into sodium and chlo- 

 rine atoms. We know this is not true, however, for sodium is an ele- 



