28 



LIFE: ITS BEGINNINGS AND NATURE 



rangement the periodic table. It is possible 

 to diagram this atomic sequence ( Fig. 2-5 ) . 

 The atomic number corresponds to the 

 number of electrons in the orbits, or the 

 number of protons in the nucleus, limited in 

 range, of course, from 1 to 98. The atomic 

 weights are arbitrary figures assigned to 

 each atom, and they depend on the num- 

 ber of protons and neutrons in the nucleus. 

 Oxygen has been assigned the figure 16 

 and others all vary in respect to it. Since a 

 single atom cannot be weighed, these fig- 

 ures must be based on measurements that 



starts a new outer orbit containing one elec- 

 tron; thus, with its two inner electrons and 

 the one in the outer orbit, it has three alto- 

 gether, giving it the atomic number 3. 

 Sodium (not shown in Fig. 2-5) is the first 

 atom that has a third ring, with 11 electrons 

 in all. Usually the inner ring must be com- 

 pleted before the next one is formed. Fol- 

 lowing this principle the numbers of pro- 

 tons, neutrons, and electrons of nearly all 

 98 elements have been determined. 



When the number of electrons in the 

 outer orbit is less than half the total num- 



H4O 

 melccut« 



oxygzn a¥om 



2 ekctronft 



ion 

 (anion) 



Fig. 2-6. A possible explanation for the formation of a molecule of water from two atoms of 

 hydrogen and one of oxygen. Note that the hydrogen ion is a naked proton, and the 

 oxygen ion is formed by the addition of two electrons. The union of the two kinds of 

 ions produces the molecule of water. 



include a great many individual atoms, 

 hence are not absolute figures. 



There are seven possible concentric rings 

 of electrons among all of the atoms, the 

 one closest to the nucleus containing two 

 electrons, and each of those beyond hav- 

 ing varying numbers. Hydrogen has one 

 proton and one electron but contains no 

 neutrons. It has the atomic number 1. 

 Helium, the next in the series, has two elec- 

 trons and, therefore, the atomic number is 

 2. However, it has an atomic weight of four, 

 because it also has two neutrons in its 

 nucleus along with the two protons. Lithium 



ber it can hold, it may lose them, or if it 

 has more than half, it may gain others to 

 complete the ring. Any change in these 

 numbers of electrons changes the electrical 

 nature of the atom; if it gains electrons it 

 becomes negative, and if it loses electrons 

 it becomes positive. Whenever the atom is 

 out of balance in respect to its electrons it 

 is an ion; if it possesses an excess of elec- 

 trons it is called an anion (because if 

 placed in an electrical field it will move 

 to the positive pole, the anode); if it has 

 lost electrons it is known as a cation (be- 

 cause it moves to the negative pole, the 



