156 FIRST YEAR COURSE IN GENERAL SCIENCE 



A single symbol, whether alone or combined with others, 

 stands for one atom, which is the name chemists have given 

 to the smallest portion of matter which enters into com- 

 bination. For example, Na stands for one atom of sodium. 

 NaCl means that there is one atom of sodium united with 

 one atom of chlorine. 



Two or more atoms make a molecule. If the atoms are 

 all of the same kind, they form the molecule of an element. 

 If unlike, they make a molecule of a compound. H 2 (read 

 "H two") represents a molecule of the element hydrogen; 

 H 2 O (read "H two O"), a molecule of the compound water. 



Below are the names and symbols of some of the well-known 

 elements: > 



Ni silver .... Ag 



N sodium . . Na 



O sulphur . . S 



P tin Sn 



Pt zinc . Zn 



EXERCISES 



1. Why does one end of a match "light" and the other not, with 

 the same treatment? 



2. Why is iron in its various forms the most important metal at the 

 present day? 



3. Why is iron often covered with paint, zinc, or graphite? 



4. What weight of gold is there in an 18 karat gold ring weighing 

 25 g.? 



6. What properties of gold make it useful in dentistry? 



6. Compare the weights of one cubic centimeter of gold and of one 

 cubic centimeter of a 12 karat gold-and-copper alloy. 



7. In ancient times, a goldsmith used an alloy in making an article 

 that was supposed to be made of pure gold. A philosopher detected 

 the cheating by weighing the article in air and then in water. Explain 

 his method. 



8. 1,000 cu. cm. of hydrogen gas weigh about yy of a gram. What 

 is the weight of an equal volume of air? 



