RADIOISOTOPES — AEBERSOLD 



221 



DEFINITIONS 



Webster reminds us that the word "isotope" comes from two Greek 

 words, "iso" and "topos," meaning "same"' and "place." The word 

 "isotope" was chosen to describe certain atoms which, although differ- 

 ent in weight, still occupy the same place in the periodic table of ele- 

 ments. Since they are atoms of the same element, they will behave 

 alike chemically, their difi'erences being only in physical properties. 

 Isotopes, therefore, are like twins that look and act alike but that are 

 different in weight. Radioactive isotopes, or radioisotopes for short, 

 are atoms that give off radiation and disintegrate to become other 

 kinds of atoms. 



Actually, isotopes are very intimately associated with our everyday 

 lives. They are not only to be found in the laboratory but everywhere. 

 Isotopes are common in the elements around us here — including those 

 in our bodies. For example, hydrogen, the simplest and one of the most 

 abundant elements, exists naturally in two forms (fig. 1). One is 



WHAT AN 



IS.... 



HYDROGEN ATOMS CAN 

 HAVE SEVERAL FORMS 



NATURAL OCCURRING NATURAL OCCURRING MAN-MADE 



RiCTHOH-jiai'''^ ^ 



"<™"(A-0 /ho 



I y NEunOH 



HYDROGEN I 

 PROffUM 



I y wtunoH 



HYDROGEN 2 

 DEUfWiUM 



HYDROGEN 3 



trTtium 



o/z^if/i^tfm LY ^ atoms 



isotopes 



MAN-MADE 



NATURAL OCCURRING NATURAL OCCURRING 



CARBON 10 CARBON 11 CARBON 12 CARBON 13 CARBON 14 



PROTONS 6 PROTONS 



NEUTRONS _4 NEUTRONS 



MASS NO 10 MASS NO. 



6 PROTONS 

 _5^ NEUTRONS 



11 mass no. 



6 PROTONS 

 _6 NEUTRONS 



12 mass no 



6 PROTONS 6 



_7 NEUTRONS _8 



13 MASS NO. 14 



Figure 1. — The word "isotope" is used to distinguish different-weight atoms of the same 

 element. The simplest of the elements, hydrogen, has three isotopes. Two of them — 

 hydrogen having a unit weight of 1, called protium, and hydrogen having a unit weight 

 of 2, called deuterium — exist in all naturally occurring hydrogen in the respective con- 

 centrations of 99.985 percent and 0.015 percent. Hydrogen 3 can be made by man in 

 the nuclear reactor although it does not occur in nature. All other elements have at 

 least three isotopes and some have considerably more. The element xenon, for example, 

 has 24 known isotopes. A total of more than 1,000 isotopes have been identified to date. 

 Pictured with the isotopes of hydrogen are the isotopes of carbon. 



