DETERMINATION OF ATOMIC WEIGHTS. 105 



7. DETERMINATION OF ATOMIC AND MOLECULAR WEIGHTS. 1 



Determination of atomic weights by chemical decomposition. 

 The great difficulties originally encountered in the determination of 

 atomic weights cannot well be described here. Consideration will be 

 given alone to the three principal methods at present in use. These 

 methods depend either on chemical action or on physical properties. 



One of the chemical methods used for the determination of atomic 

 weights depends upon the determination of the proportions by weight in 

 which the element, the atomic weight of which is unknown, combines 

 with an element the atomic weight of which is known. For instance : 

 If in decomposing a substance we find it to contain in 72 parts by 

 weight, 16 parts by weight of oxygen 2 and 56 parts by weight of 

 another element, we have a right to assume the atomic weight of this 

 second element to be 56, provided, however, that the compound is 

 actually formed by the union of one atom of oxygen and one atom of 

 the other element. These 56 parts by weight might, however, repre- 

 sent 2, 3, or more atoms. If 56 represented 2 atoms, the atomic 

 weight would be but 28 ; if 4 atoms, 14. 



As this mode of determination gives no clue to the number of 

 atoms present in the molecule, the results obtained are liable to be 

 incorrect. In fact, the atomic weights of a number of elements had 

 originally been determined incorrectly by using the above or similar 

 methods, and many of these old atomic weights had to be changed 

 (generally doubled) in order to obtain the correct numbers. 



Thus, in examining water, it was found that it contained 8 parts 

 by weight of oxygen to 1 part of hydrogen, and the conclusion was 

 drawn that the atomic weight of oxygen was 8, and that the molecule 

 of water was formed by the union of one atom of hydrogen and one 

 atom of oxygen. It will be demonstrated below why we assume to- 

 day that the atomic weight of oxygen is 16, and that the molecule of 

 water is composed of 2 atoms of hydrogen and 1 of oxygen. 



Another chemical method of determining atomic weights is the 

 replacement of hydrogen atoms in a known substance by the element 

 the atomic weight of which is to be determined. For instance : Hy- 

 drochloric acid is composed of one atom of chlorine weighing 35.2, 

 and one atom of hydrogen weighing 1, the molecular weight of hy- 

 drochloric acid being 36.2. If in this acid the hydrogen be replaced 

 by some other element, for instance by sodium, we are enabled to 

 determine the atomic weight of sodium by weighing its quantity and 



1 The consideration of Chapter 7 should be postponed until the student has become famil- 

 iar with chemical phenomena generally. 



2 For purposes of discussion, whole numbers are often used in place of exact atomic weights 

 when these contain decimals. 



