FOUNDATION FACTS AND PRINCIPLES 5 



atom has four hands (=C=). In hydrogen molecules the atoms 

 are always in pairs (H H or Hj). Thus the weight of the hydro- 

 gen molecule is two, because it contains two of the unit atoms. The 

 atom of oxygen weighs sixteen times as much as the hydrogen 

 atom, consequently 16 is the atomic weight, or the combining 

 weight, of the element oxygen. The molecule of ordinary oxygen 

 contains two atoms (O =O or Og) , but there is a form of oxygen, 



called ozone, which contains three atoms in the molecule (O O 



or Og). The molecular weight of ordinary oxygen is 32, while the 

 molecule of ozone weighs 48 times as much as one hydrogen 

 atom. One oxygen atom has power to hold two hydrogen atoms 

 (H O H or H 2 O). This is a compound which might be 

 called dihydrogen oxid, but which is commonly called water. The 

 molecular weight of water is 18, the sum of the atomic weights. 

 Separately, hydrogen and oxygen are both gases under ordinary 

 conditions, but the compound H 2 O possesses different properties, 

 being a liquid at ordinary temperatures, although water becomes 

 a gas at high temperature and a solid at low temperature. 



While it is common knowledge that this compound exists in 

 three forms, solid, liquid, and vapor, and that it is easily changed 

 from solid ice to liquid water and from liquid to vapor (steam), 

 it is not so generally known that most elements and most com- 

 pounds may exist in each of these three forms under proper condi- 

 tions of temperature and pressure. 

 One atom of carbon may combine with four atoms of hydrogen, 



/Hv /H x 



forming the gas compound I XY or CH 4 ) called methane or 



V H/ \H 



marsh gas, a constituent of illuminating gas, and sometimes formed 

 in stagnant marshes. This hydrocarbon has a molecular weight 

 of 1 6 and is the lowest in a very large series of compounds contain- 

 ing only hydrogen and carbon. One atom of carbon with its four 

 bonds may unite with two atoms of oxygen, forming carbon dioxid 

 (O = C = O or CO 2 ) , a chemical reaction which occurs in the com- 

 bustion of coal or other substances containing carbon, as already 

 explained. 



Monovalent atoms have one bond, or one hand (mono means 



