16 LIFE: ITS NATURE AND ORIGIN 



atom be represented by a period on this printed page, its satellite 

 electron would be represented by a small pea about 50 miles 

 away. Some notion as to the immensity of the numbers of atoms 

 and molecules is given by this estimate of Professor Herbert 

 Freundlich: Imagine each water molecule in a liter of water 

 marked for identification, and the whole lot uniformly mixed 

 throughout all the waters of all the seas and oceans, down to the 

 lowest depths. Suppose, thereafter, that a liter of water is taken 

 at random, anywhere in the oceans of the world; it would contain 

 approximately 25,000 of the original marked molecules. 



Isotopes — a New Concept of the Elements 



Shortly before World War I, Sir J. J. Thomson developed the 

 mass spectrograph, an instrument in which positively charged ions 

 of elements (or of compounds) in motion are subjected to the pull 

 of magnetic or electric fields and are diverted from their course 

 in proportion to their mass. On examining (1912) the rare gas 

 neon, he found that it gave two parabolas, one corresponding to 

 particles with a mass of 20, the other to particles with a mass of 

 22; that is, neon has two kinds of atoms. In 1919 Francis William 

 Aston of Cambridge University (Nobel prize, 1922) using im- 

 proved apparatus, confirmed Thomson's work and found that most 

 elements have atoms of different atomic weights, though all atoms 

 of the same element have the same chemical properties. 



Thus fell another dogma of chemistry — the notion that all 

 atoms of each element are precisely alike. Even in the cases of 

 the relatively few natural elements where this is so (e.g., beryllium, 

 fluorine, sodium, argon, phosphorus, manganese, cobalt, arsenic, 

 iodine, molybdenum, and gold), new man-made radioactive iso- 

 topes, usually of transient life, have been created. 



Some of our natural elements have quite a number of isotopes, all 

 chemically the same because they have the same net positive nuclear 

 charge and therefore the same number of planetary electrons; but the 

 masses of the isotopes may differ considerably. It is the exterior 

 electron rings which determine the chemical properties of an atom, 

 recalling the saying that beauty is only skin deep. In the case of the 

 metal tin, nine radioactive isotopes have been made synthetically, 

 with half-lives running from 9 minutes to 400 days; but the natural 

 metal consists of ten isotopes whose atomic weights range from 112 

 to 124. 6 Chemists use the average, 118.70 as the atomic weight of tin; 

 for though the mass of the individual atoms varies, the natural average 

 is practically constant, and all the atoms act chemically like tin. 



