70 - The Cell 





HYDROGEN 

 H 



•V- 



HELIUM 

 He 



( © ) 



LITHIUM 

 Li 



BERYLLIUM 



BORON 



(<8» 



CARBON 

 C 



NITROGEN 

 N 



OXYGEN 

 



(®; ; 



FLUORINE 

 F 



'•©}i) il{©}], 



NEON 

 Ne 



•@« 



r®/ 



SODIUM 

 Na 



•©•.; 



MAGNESIUM 

 Mg 



•©•, 



Fig. 4-4. Structure of atoms of atomic numbers 1-20 and 26. The central circle represents the atomic nucleus, 

 the number in the circle showing the net positive charge, i.e., the atomic number; the small black circles rep- 

 resent planetary electrons. These figures are purely diagrammatic and do not pretend to show the actual spatial 

 configuration of the electrons. The atoms whose symbols are printed in heavy type are those of elements generally 

 present in living matter. 



negatively charged particles, the electrons, also 

 present in the atomic center. 4 Energetically re- 

 volving around the atomic center, like the planets 

 of the solar system, there is another group of 

 electrons, the planetary electrons. The number 

 of protons and electrons present in the whole 

 configuration differs for each particular kind ol 

 atom (Fig. 4-4). but always the total number of 

 electrons present in a given atom is exactly equal 

 to the number of its protons. In short, each atom, 

 as a whole, is an uncharged body. 



A knowledge of atomic structure provides an 

 understanding of how various atoms enter into 

 chemical union. One group of the atoms (the 

 electropositive atoms) tends to give up one or 

 more planetary electrons, transferring the elec- 

 trons to the planets of atoms of another group 

 (the electronegative atoms), which have a tend- 

 ency to gain planetary electrons. When such an 

 exchange occurs (Fig. 4-5), the participating atoms 

 tend to cling together, constituting a molecule. 

 The atom that has lost electrons now possesses 

 a positive charge, and this makes it cling to the 

 other atom, which having gained electrons, pos- 

 sesses a negative charge. Such studies also explain 



4 Actually the atomic center is composed mainly of 

 neutrons, each formed by the union of one proton 

 with one electron. In fact, the number of free protons 

 present in the atomic center is restricted to the num- 

 ber of planetary electrons in each atom, and this de- 

 termines the atomic number (Fig. 4-4). 



SODIUM 

 ATOM 



CHLORINE 



ttflW 



SODIUM ' . 



# /CHLORIDE 

 ION 



o= 



SODIUM CHLORIDE 



MOLECULE „,«, 



Fig. 4-5. Chemical union between an atom of sodium 

 (Na) and an atom of chlorine (CI) to form a molecule 

 of salt (NaCI). 



the existence of a third general group of atoms, 

 the inert elements, like helium and neon (Fig. 

 4-4). These inert atoms never form any chemical 

 compounds. Each possesses a very stable planetary 

 system, which resists the gaining or losing of elec- 

 trons. But among chemically active atoms, the 

 number of planetary electrons that can be gained 

 or lost during chemical union is specified as the 

 valence of the particular atom in question. 



