16 



THE FOUNDATION 



Part I 



containing a heavy isotope of hydrogen also distills. Isotopes of uranium are 

 not procured by any of the easier methods; skill, persistence, and elaborate 

 equipment are required. 



Isotopes are also put to various uses, in war, in biological investigation, and 

 in medicine. The atomic bombs of the Second World War contained isotopes 

 of heavy atoms with unstable nuclei that flew apart establishing chain ex- 

 plosions of tremendous destruction. The political condition of the world has 

 established an association of isotopes and war. There is hope that this may 

 sometime give place to great constructive uses. To the world at large, atomic 

 bombs have almost hidden the importance of the radioactive isotopes that are 

 being used as tracers in living plants and animals. 



Hydrogen atom 



Deuterium atom 



Fig. 2.6. Diagrams of the structural plan of the atom. As they 

 are at this date generally named the particles inside the nucleus 

 are: the protons ( + ) that carry positive charges and the neu- 

 trons (0) that carry no charges; the electrons outside the nucleus 

 bear negative charges. Hydrogen atoms have one proton and one 

 electron. Deuterium atom, an isotope of hydrogen (heavy hy- 

 drogen) consists of a nucleus with one proton and one neutron, 

 and a single electron moving around it. Helium atom, the 

 nucleus consisting of two protons and two neutrons, has two 

 electrons moving around it. Helium gas is used in dirigible bal- 

 loons. 



Helium atom 



Ions. Atoms may gain or lose electrons and are then known as ions. If elec- 

 trons are lost, the ion is positively charged; if they are gained, it is negatively 

 charged. Ions combine more readily than electrically neutral atoms. Water 

 facilitates the splitting of substances into ions. Living organisms are largely 

 water and many substances are present in them chiefly in a dissolved state. In 

 solution many of these dissolved substances split into simpler ones and ions 

 are formed (Fig. 2.7). When crystals of common salt (sodium chloride, 

 NaCl), a component of the blood of all animals, dissolve in water, the ions of 

 the sodium (Na+) already present in the lattice of the crystal are separated 

 by the attraction of the polar molecule of water. The crystal framework is thus 

 broken and the ions are free in the solution. Their formation in salt solution 

 is expressed by the formula, NaCl = Na+ -f CI-. 



Because of the positive and negative charges of ions, the living body can 

 conduct electricity. When the opposite poles of a battery are placed in water, 



