some Carbonates of the Calcite Group. 355 



two negative charges. That the attracting forces are not, 

 however, preponderantly the electrostatic forces arising 

 from these charges is shown by the decrease in size of the 

 unit of structure when the presumably larger manganese 

 atoms take the place of calcite atoms. 



From the point of view of the present knowledge of 

 the structure of the atom and the electronic nature of 

 chemical bonds, calcium carbonate (or manganese carbon- 

 ate) can be simply represented in one of two ways. The 

 determination of crystal structure has shown that all 

 three oxygen atoms must be similarly related to a carbon 

 atom. The neutral atom of carbon with an outside 

 cluster of four electrons tends to add on four more 

 and close its "cluster" of eight. 37 Oxygen, with six elec- 

 trons in the outermost "ring", has a still greater tendency 

 to acquire two electrons, while calcium readily loses its 

 two outside electrons. If one is willing to believe that the 

 atoms of oxygen are able to acquire completely the four 

 electrons from carbon as well as the two from the metal 

 atom, leaving them to carry four and two positive charges 

 respectively while each oxygen atom is doubly negative, 

 then calcium carbonate must be represented somewhat as 

 in (A), fig. 17. If this is the true state of affairs the dis- 

 tance from carbon to oxygen would have been different in 

 calcium from that in manganese carbonate. If, as is per- 

 haps a more probable arrangement, the carbon and oxy- 

 gen atoms are held together by sharing electrons, the 

 representation would be according to (B) fig. 17. 38 In 

 this second possibility the carbonate group and the metal 

 atom may be charged. 



If all the atoms in the crystal are assumed to be elec- 

 trically charged, then the outside electrons may, but need 

 not, be thought of as arranged at the corners of a cube. 39 

 If, on the other hand, they are not all of them "ions ' ', then 

 a cubical arrangement of the outside electrons of the 

 atoms is in this case impossible. 



37 1. Langmuir, J. Am. Chem. Soc, 41, 868, J 919; E. W. G. Wyckoff, 

 op. cit. 



38 In this figure the outside elections are represented by black circles. 

 In B the two electrons represented as close to the carbon atom would be 

 symmetrically placed on the axis of the group, at equal distances above 

 and below the plane of the paper. 



39 1. Langmuir, op. cit. 



