ELEMENTS AND COMPOUNDS 13 



that there are two molecules of water. Figures placed to 

 the left of a formula and on the same line indicate the 

 number of molecules, while figures to the right of the in- 

 dividual element represent the number of atoms of ele- 

 ments in each molecule. Hence the formula of a com- 

 pound always designates the composition of the molecule, 

 and the number and kind of atoms contained. Farther 

 study of the formulas of compounds will show that addi- 

 tional facts, as composition by weight and volume, are 

 also represented. 



Exercise. Name the elements, the number of molecules, and 

 the number of atoms in each molecule in the following formulas : 

 NaCl, CaCl 2 , 2KC1, 2K 2 SO 4 , A1 2 O 3 , sN 2 O 5 , H 2 SO 4 , NaOH, HPO 3 . 



17. Atomic Weights. An atom is the smallest part of 

 an element present in a molecule. Atoms have definite 

 properties, as weight. Hydrogen is the lightest material 

 known. An atom of hydrogen, or the smallest part of 

 hydrogen which can enter into chemical combination, is 

 considered as having a weight of i . The weight of the 

 atom of any element is the number of times heavier that 

 atom is than hydrogen, which is the standard. Oxygen, 

 for example, has an atomic weight of 16, that is, an atom 

 of oxygen weighs 16 times as much as an atom of hydro- 

 gen. Carbon has an atomic weight of 12, that is, an 

 atom of carbon is 1 2 times as heavy as an atom of hydro- 

 gen. The way in which the atomic weights are obtained 

 cannot, at this stage of the work, be profitably considered. 

 Atomic weights are, however, obtained with a high de- 

 gree of accuracy, and while the individual atoms and 

 molecules are not susceptible, at the present time, to sep- 



