SECTIONAL COMMUNICATIONS 
ORDERED BY THE GENERAL COMMITTEE 10 BE PRINTED in eatenso. 
DISCUSSION ON THE STRUCTURE 
OF MOLECULES. 
Dr. Invinc Lancmurr opened the discussion with an account of the theory 
proposed by G. N. Lewis in 1916, and subsequently extended by the speaker, 
according to which the fundamental conceptions of valence and the numerical 
values of positive and negative valence and covalence may be derived for 
most elements from a few postulates regarding the structure of atoms. 
Assuming the Rutherford type of atom, consisting of a positive nucleus sur- — 
rounded by a number of electrons equal to the atomic number of the atom, 
and also assuming that Coulomb’s law applies to the forces between the charged 
particles in the atom, the existence of repulsive forces must also be recognised. 
These forces prevent the electrons from falling into the nucleus, but it is 
immaterial for the purpose whether the repulsive force be dynamic, as assumed 
by Bohr, or static, as assumed by Lewis and by the speaker. Only three 
postulates have to be made. 
Postulate 1.—Z'he electrons in atoms (or ions) tend to surround the nucleus 
in successive layers containing 2, 8, 8, 18, 18, and 32 electrons respectively.— 
When the number of electrons is such that they cannot all form into complete 
layers in accord with Postulate 1, the extra electrons remain in the outside 
incomplete layer, called the sheath of the atom. Every electrically neutral 
atom must contain a number of electrons equal to the atomic number of the 
nucleus. Should the outside layer be nearly complete, a few additional electrons 
may be taken up to complete the layer, forming a negatively charged ion. The 
sheath of any atom or atomic ion consists of all the electrons in the outer 
layer, provided that this layer is incomplete when the atom is electrically neutral. 
The inert gases are the only elements whose neutral atoms have no sheaths, 
since their outer layers consist of electrons which already form a complete layer 
in the neutral atom. Sodium and calcium ions also have no sheaths, but the 
atoms of those metals have an incomplete sheath containing one electron. The 
‘fluorine atom has an incomplete sheath of seven electrons, whilst the fluorine 
ion has a complete sheath of eight electrons. The tendency expressed by 
Postulate 1 can only be satisfied by an interaction between atoms involving a 
rearrangement of electrons. This is to be regarded as the fundamental cause 
of chemical action. A complete compound is formed if the interaction between 
atoms leads to the complete satisfaction of the postulate. 
Any pair of electrons which is rendered stable by the proximity of one or 
more positive charges is called a duplet. Postulate 2 states that two atoms 
may be coupled together by one or more duplets held in common by the com- 
pleted sheaths of the atoms. A given group of neutral atoms may interact to 
complete their sheaths in two ways: (1) Atoms having sheaths containing only 
a few electrons may give up these extra electrons to other atoms, and atoms 
having nearly complete sheaths may take up electrons from other atoms. 
(2) Atoms may share duplets with other atoms, and so complete their sheaths 
with fewer electrons than would otherwise be necessary. The transfer of 
electrons corresponds with positive and negative electrovalence, whilst the 
sharing of duplets corresponds with covalence. The term electrovalence covers 
both positive and negative valence, which differ only in algebraic sign, being 
positive when an atom gives up electrons and negative when it takes up 
electrons. The electrovalence of an atom in a compound may thus be defined 
as the number of electrons which the neutral atom must give up in forming 
that compound. If the neutral atom -must take up electrons, the electro- 
valence must be expressed as a negative number. Defining the covalence as 
the number of duplets which an atom shares with neighbouring atoms, every 
duplet shared by two atoms corresponds with a (covalence) bond between atoms. 
A simple algebraic relation shows that the sum of the electrovalences and the 
covalences for all the atoms in any complete compound is zero. 
