THE STRUCTURE OF MOLECULES. 469 
The kernel of an atom is defined as that part of the atom which remains 
after the sheath is removed. Since the neon atom has no sheath, the whole 
atom constitutes a kernel with zero charge. The kernel of the sodium atom 
is the sodium ion with a single positive charge, whilst the kernel of the fluorine 
atom is the fluorine ion, consisting of the nucleus and two electrons, the whole 
having seven positive charges. The positive charge on the kernel is equal to 
the number of electrons in the sheath of a neutral atom. Assuming that 
duplets are shared equally by two atoms, which would be the case if the two 
atoms were substantially alike in size and structure, the sum of the electro- 
valence and the covalence, for any atom in a compound, is equal to the residual 
atomic charge. 
Postulate 3.—The residual charge on each atom and on each group of atoms 
tends to a minimum.—By ‘ residual charge’ is meant the total charge regardless 
of sign. By ‘group of atoms’ is meant any aggregate of atoms characterised 
by nearness to one another. It will be possible to express this as a quantitative 
law when the repulsive forces between charged particles are better understood. 
As a familiar example, in any small finite volume of a salt solution the charges 
on the positive and negative ions tend to be equal, or the residual charge 
tends to a minimum. Postulates 1 and 3 are often in conflict, and the result 
is then a compromise, complete compounds being formed provided that this 
can take place without the charges on the ions becoming too large. Although 
Postulate 3 does not definitely fix the charges of the individual atoms in the 
compounds under consideration, it does determine the distribution of these 
ions in space. This is a factor of prime importance in the crystal structure, 
in the electrolytic conductivity of substances in the liquid state, and in other 
properties. When the number of ions of one sign is much larger than that of 
the other sign, as in such compounds as AICI3, PCls, or SF, Postulate 3 requires 
that the negative halogen atoms shall surround the most strongly positive 
atoms. The ions thus form groups having strong internal and weak external 
fields of force, so constituting molecules of considerable stability and inertness. 
This is in accord with the volatility and absence of electrolytic conductivity of 
these compounds. 
Salts are typically complete compounds, and when the atomic charges are 
small, as in NaCl, BaBr, K,S, &c., the compounds are fairly readily fusible, 
soluble in liquids of high dielectric constant, good electrolytic conductors when 
molten or in solution, and volatile with great difficulty. With larger charges, 
as in MgO, BN, Al,0,, &c., the strong forces give great infusibility, insolubility, 
and hardness. These compounds are good electrical insulators at moderate 
temperatures, but conduct electrolytically when molten. 
In any group of atoms Postulates 1 and 3 are both completely satisfied if 
the covalence of each atom is equal to the negative valence of that atom. The 
negative valences of carbon, nitrogen, oxygen, and sulphur are 4, 3, 2, and 2 
respectively, whilst that of hydrogen and the halogens is one. On writing 
structural formule as employed in organic chemistry, using these values for 
the valence, results in complete accord with Postulates 1, 2, and 3 are obtained. 
It is seen that only negative valences can be used in such structural formule 
(that is, as covalences), and that even these can only be legitimately used in 
compounds from. which electropositive atoms are entirely absent; for if some 
of the atoms have a positive residual charge, some others must have a negative 
charge, and for these as well as for electropositive atoms the covalence is not 
equal to the negative valence. From this point of view it is incorrect to write 
such a structural formula as 
H—O O 
N\A 
Na — Cl, s 
/N\ 
H—O O 
in which the covalence of one atom is taken as equal to the positive valence 
of that atom. In crystalline salts only electrovalences are concerned. The 
sodium and chlorine atoms in sodium chloride are converted into ions by the 
passage of an electron from one atom to the other. There is no definite molecule, 
and each sodium jon is similarly related to six chlorine ions. The ready 
ionisation of the salt when dissolved in water is in accord with this view. 
