370 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 





Figure 5. 



eight electrons and in this way the outer shells are filled, as is shown 

 diagrammatically in figure 5. Bonding such as this, where a valence 

 electron from one atom is shared with the adjacent atom, is called 

 covalent bonding. It is unportant to notice that all the valence elec- 

 trons are held in the bonds and none are free to move. This kind of 

 tetragonal structure is common to diamond, germanium, silicon, and 

 certain forms of tin. 



Wo consider next why the diamond has such great cohesive strength 

 and why silicon and germanimn are much softer although their crys- 

 tal structures are the same. A comparison of the physical properties 

 of these three substances is shown in table 1. 



