552 SMITH'S INTERMEDIATE CHEMISTRY 



atomic weights exactly 79 and exactly 81. Mercury (at. wt., 

 200.6) appears to exist in as many as six forms, with atomic 

 weights ranging from 197 to 204. Other elements, however, such 

 as oxygen, nitrogen, and iodine, give no indications of isotopes. 

 The fundamental atomic weights obtained by Aston are with 

 one exception whole numbers, within the limits of experimental 

 error. The single exception is hydrogen (at. wt., 1.008). 



Atomic Structure. On the basis of the above results, general 

 theories of atomic structure have been built up by Harkins and 

 Rutherford, postulating hydrogen and helium atoms as the " bricks " 

 building up the atoms of all elements. The elements with atomic 

 weights divisible by 4 are considered as constructed entirely of 

 charged helium nuclei, with surrounding electrons; thus C = 

 3He++ + 6e, = 4He++ + 8e, etc. Other elements must be 

 assumed to contain hydrogen atoms also in their structure (see 

 p. 17). The decrease in the mass of the hydrogen atom from 

 1.008 in hydrogen itself to exactly 1 in all other atomic types 

 has been ascribed to a " packing effect." 



Valence and Atomic Structure. The electrons surrounding 

 the nucleus are arranged, according to a theory recently developed 

 by Lewis and Langmuir, in successive concentric shells. The 

 total number of electrons in these shells must be equal, since the 

 atom as a whole is electrically neutral, to the number of free pro- 

 tons in the nucleus, in other words, to the atomic number. 



The case of the hydrogen atom (atomic number = 1) has already 

 been considered (p. 195). The helium atom (atomic number = 

 2) has two electrons, which are supposed to be situated on opposite 

 sides of the nucleus a very stable arrangement. 



No more electrons can be contained in the first shell, hence in 

 succeeding elements the additional electrons begin to build up a 

 second outer shell. Only the electrons in this outer shell can be 

 added to or lost in interactions with other atoms (see p. 217), 



