99 



be equal to the excess of protoiies of tlie nucleus. The elections 

 which can more or less easily be shifted, and can even be removed, 

 are found in the outer .shell, and also electrons of other atoms can 

 penetrate into this outer shell. 



There is further a general tendency to gather eight electrons in 

 this outer shell, because this represents most likely a very stable 

 condition of equilibrium. We meet with this constellation in the 

 noble gases, which do not possess chemical aflinity. Only helium 

 has only two electrons in its outer shell, and evidently forms an 

 exceedingly stable whole with the nucleus. 



The mono-valent metals have only one electron in their outer shell, 

 and will easily split this off, in this way getting into the condition 

 of the nnlli-valent element, which stands one place lower down as 

 to its rank; the elements of the seventh group, the halogens, have 

 seven electrons in the outer shell, and will have a tendency to 

 add one electron, passing with it into the condition of the nnlli- 

 valent element, which is one place higher in rank. 



Thus an exceedingly stal)le substance of the type of Helium-Neon 

 will be formed when Li and F are joined, with this ditierence that 

 there exists a very strong electric field between these atoms, which 

 is wanting in the noble gases. 



KossEi, has designated this kind of bonds by the name of hetero- 

 polar, they exist between all metallic elements on one side and the 

 non-metallic ones on the other side. When the electron of the metal 

 has entered the shell of the non-metal, this has obtained for the 

 metal-ion a same value as the seven already present ones, which 

 means that the metal-ion is no longer bound to a definite place in 

 the molecule; it can place itself opposite to each of the electrons 

 present. 



When the number of electrons in the outer shell increases, resp. 

 decreases, they no longer get so easily quite outside, resp. the 

 power to absorb foreign electrons has diminished ; then ensues an 

 interpenetration of the two shells, in which one electron of each 

 of the atoms joins to a pair in the mutual shell division. 



This is the homöopular bond according to KossEi,, in which the 

 two atoms are bound to a very definite place. The hetero-polar or 

 briefly polar bond gives rise to molecules which conduct the electric 

 current e. g. in aqueous solution ; the homöo- or non-pohir bond is 

 met with in substances that do not conduct the electric current. 



As a type of the first we may name the alkali-halogenides, as a 

 type of the second the orgaiuc compounds, but also water, boron- 

 trichloride etc. 



7* 



