28 Introduction 



Compounds. — The way in which atoms combine to form com- 

 pounds is precisely determined by the number of electrons in the outer 

 orbit. The molecules resulting from the combining of atoms are ap- 

 parently held together by the sharing of electrons. 



Better to explain the above, the formation of sodium chloride can 

 be considered. Sodium with 1 1 electrons in its orbits has 2 in the inner- 

 most shell, 8 in the second, and but 1 in the outer. Chlorine has 2 

 electrons in its innermost ring, 8 in the next and 7 in the outermost 

 one. It lacks a single electron to complete this ring of 8. Since sodium 

 has but one in the outer ring, one atom of sodium will satisfy one 

 atom of chlorine. Sodium chloride (common table salt) is the re- 

 sulting compound. 



It is well to note that this compound has properties quite unlike 

 those of the elements from which it is formed. Sodium is a white 

 metal that burns readily and reacts violently; chlorine at normal tem- 

 peratures is a highly poisonous gas. In combination they produce 

 sodium chloride which is not toxic and which is a necessity to liv- 

 ing things. 



An atom which is completely saturated in its outer ring will not 

 unite with other atoms under ordinary conditions. The gas, helium, 

 with its saturated outer ring is a very inert material. Hydrogen 

 with but a single electron unites easily to form a large number of 

 compounds. Hydrogen and oxygen readily combine to form water, 

 one of the commonest and most essential compounds found in all living 

 things. Oxygen with 6 electrons in its outer orbit needs two more for 

 completion. Two atoms of hydrogen ordinarily combine with one of 

 oxygen to form a molecule of water. 



Elements always combine with one another in definite proportions 

 which are determined by the number of electrons in the outer orbit 

 of each element. 



Characteristics of Elements. — Elements all have characteristic 

 atomic numbers, atomic weights, and symbols. They also possess 

 specific melting and boiling points as well as unique properties of color, 

 chemical behavior, etc. The atomic weight, as pointed out above, is 

 determined by the number of neutrons and protons in the nucleus; 

 properties such as the melting and boiling points are determined by 

 the total number of protons, neutrons, and electrons, while the chem- 

 ical properties are determined by the number of electrons in the outer 

 orbit. The symbol of the element is the abbreviation of the Latin 

 name. 



