256 PHENOMENA, ATOMS, AND MOLECULES 



The manganese and beryllium atoms are thus quadricovalent, the water 

 and ion share single pairs of electrons with the central atom. In the first 

 compound the oxygen is tercovalent. The substitution of water or ammonia 

 for fluorine or oxygen in these compounds is analogous to the substitution 

 of K^ in K2SO4 by NU^\ 



The same explanations apply to the other examples cited by Barker. 

 Those shown in Group K are of interest because in siHcates we usually 

 have to deal with compounds in which no pairs of electrons are held in 

 common between atoms. Thus in the octet equation 2p = Sn — e we place 

 p = O and find e — 8w. In silicates, oxygen is the only element which 

 forms octets, so n is equal to the number of oxygen atoms. This condition 

 (e = 8w) is practically the only valence condition that needs to be fulfilled 

 by silicates, and it is thus a complete statement of the valence theory for 

 these compounds. The isomorphism of the two compounds of Group K 

 is consistent with the octet theory since in both compounds the number 

 of atoms is alike (13) and so is also the number of available electrons, 

 e = 8n = 64. 



SUMMARY 



The octet theory of valence indicates that if compounds having the 

 same number of atoms have also the same total number of electrons, the 

 electrons may arrange themselves in the same manner. In this case the 

 compounds or groups of atoms are said to be isosteric. Such compounds 

 should show remarkable similarity in physical properties, that is, in those 

 properties which do not involve a separation of the atoms in the molecule. 



Table I gives a list of various isosteres predicted by the octet theory. For 

 example, O""; F'; Ne, Na"" and Mg*'' are isosteric. Other examples are 

 No— CO— CN-; CH4— NH/; N3-— CNO'; CIO4-— SO4"— PO4"- ; 

 NO3" — CO3"', etc. In cases where isosteric groups have the same electric 

 charges (isoelectric) their properties are directly comparable; thus No and 

 CO; N2O and CO2; KN3 and KNCO, etc., are nearly alike (in pairs) 

 in all their physical properties. But when the charges are unlike the 

 similarity may manifest itself between properly chosen compounds ; thus, 

 according to the octet theory, we should expect sodium nitrate and calcium 

 carbonate to have similar constitutions and therefore to have similar 

 crystalline forms, as is in fact known to be the case. 



The following cases of crystalline isomorphism are thus predicted by 

 the theory and are found to exist according to published crystallographic 

 data: NaF— MgO ; KN3— KNCO; KNO3— SrC03; KCIO4— SrS04; 

 NaHS04— CaHP04; MnSe04.2H20— FeAs04.2H20, etc. The following 

 cases are predicted by the theory but cannot yet be tested because of lack 



