Chemistry was in bad shape at the beginning of this century. Most
of the knowledge of chemical properties that we have covered in
the first six chapters was available, but without the underlying
models that would tie it together.
The usefulness of concepts of atoms, molecules, and moles was recognized,
but there were still respectable chemists who doubted whether atoms
had a real existence other than as conceptual models to account
for observations. The periodic table organized chemical properties
in a sensible manner, and had led to the successful prediction of
properties of new elements, but no one knew why the table had the
structure that it did. Ideas about the shapes of molecules were
in their infancy.
Van't Hoff used the number of different substituted methane compounds
that could be made to argue for tetrahedral bond geometry around
carbon atoms, but there was no theory of chemical bonding to explain
why this should be true.