54 SCIENCE PROGRESS 



extra electron is taken up. The outer shell now resembles that 

 of the inert atom of neon, with the difference that within there 

 are only 7 positive charges instead of 8. Consequently the 

 fluorine atom becomes negatively charged, while the lithium 

 atom is positively charged, having given up its electron. In 

 other words, the fluorine and lithium " ions " are held together 

 by electrostatic forces. There will be a free external field, and 

 lithium fluoride is a solid body, building up a space lattice like 

 metallic lithium. It is a non-conductor because, every octet 

 being completed, there are no free electrons. On melting, or 

 solution, it becomes a conductor, since the ions can move 

 under the action of an electromotive force. 



Alternatively, an atom of fluorine can complete its octet 

 by sharing a pair of electrons with another atom of the same 

 or another kind. If we think of the electrons in the outer shell 

 as occupying the corners of a cube, the molecule of fluorine 

 may be pictured as two cubes with an edge in common, two 

 electrons doing duty in both cubes. The molecule is again 

 very stable, as exemplified by the very low boiling-point of the 



This view of chemical combination as the sharing of electrons 

 leads to Langmuir's octet theory of covalence. The covalency 

 of an atom is the number of pairs of electrons it shares with 

 other atoms. If we represent by e the total number of available 

 electrons in the outside shells of the atoms forming a given 

 compound, and let w be the number of octets formed, holding 

 p pairs of electrons in common, we see that, for every pair 

 of electrons shared, there is a saving of 2p in the number of 

 electrons needed to form octets. We have, therefore 



g = 8w — 2p, or (i) 



p = ^{8n-e) (ii) 



Of course, electrons held by a hydrogen nucleus in common 

 with an octet must not be counted in reckoning the value of p, 

 since they do not result in any saving in the numbers of electrons 

 required to form octets. 



The use of formula (ii) will be clear if we employ it to 

 determine the structures of a few compounds. The internal 

 arrangement of water molecules is of great interest from its wide 

 application as a solvent. Since the two hydrogen nuclei always 

 tend to hold pairs of electrons and never octets, we may take 

 n = I for the oxygen atom. There are 6 available electrons 

 in the oxygen atom and 2 in the two hydrogen atoms, making 

 e = S, whence equation (ii) gives p = o. This means that no 

 electrons are held in common between octets, which is obvious 

 in this case, since there is only one octet. The two hydrogen 



