32 



PRINCIPLES OF ANIMAL BIOLOGY 



Fig. 21. — Diagrams of atoms 

 of hydrogen (left) and helium 

 (right). The central black spot 

 is the nucleus; the concentric 

 circles mark oflf the shell of 

 negatively charged electrons. 



number of electrons surrounding it. In hydrogen (H), which is the 

 simplest and lightest of the elements, the nucleus consists of just 1 proton, 

 no neutron, and the atom has just 1 electron (Fig. 21, left). The single 

 unit of positive charge furnished by the proton is neutralized by the 

 negative charge of the electron. Helium (He), has 2 protons and 2 



neutrons in the nucleus, which is therefore 

 four times as heavy as the hydrogen 

 nucleus, but it bears only twice as great a 

 positive charge. To balance this positive 

 charge, there are 2 electrons in the atom 

 (Fig. 21, right). Carbon (C) has 6 protons 

 and 6 neutrons (in the nucleus) and 6 

 associated electrons; oxygen (O) has 8 

 protons and 8 neutrons in the nucleus, with 

 8 electrons'; while chlorine (CI) exists in 

 two forms, one of which has 17 protons and 18 neutrons, the other 

 17 protons and 20 neutrons in the nucleus, with 17 surrounding 

 electrons. 



The details of these particular elements are not important to the 

 biologist, but the fact that they are composed of identical kinds of 

 units and that they differ only in the number and arrangement of these 

 units should be understood. Every element has a different number of 

 protons and electrons from every other element. From the lightest 

 element, hydrogen, which has 1 

 proton and 1 electron, to the one 

 long believed heaviest, uranium 

 (U), which has 92 protons and 92 

 electrons, there should be 92 ele- 

 ments. All but two or three of 

 these have been obtained in 

 chemical laboratories. News- 

 papers occasionally report the 

 discovery of one or more of the missing elements, which await confirma- 

 tion by other investigators. In the construction of the atomic bomb two 

 elements with 93 and 94 protons, respecti\'ely, were produced. 



The chemical properties of an element, the ways in which it reacts 

 with other elements, are determined by the electrons surrounding the 

 nucleus. When these electrons are numerous, they are arranged in shells, 

 some near the nucleus, others farther away. The 2 electrons of lu^ium 

 (Fig. 21) constitute such a shell, and a similar inner shell of 2 is in all 

 elements heavier than helium. Outside this is a shell which may contain 

 from 1 to 8 electrons. Oxygen has 6 electrons in this outer shell, as 

 diagrammatically indicated in Fig. 22. When the number of electrons 



Fig. 22.- -Atom of oxjgcn; two wajs of 

 representing its two shells of electrons. 



