Page 34 - Modern physical chemistry

2

Structure in Molecules

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2. 1 Introduction

TO EXPLAIN WHY PURE SUBSTANCES COMBINE in definite proportions, Dalton about
1807 suggested that each elementary substance is made up of identical atoms and that
the atoms of different substances combine in definite ratios to form molecules. The dif-
ferent molecules of a given compound are equivalent. Presumably, each has the same
structure, the same arrangement of atoms. Both chemical and physical properties are
used to elucidate this structure.
N ow, a given amount of a pure elementary substance is neutral. So if it contains only
one kind of atom, each atom is neutral. However, rubbing the substance with a different
material causes it to become charged. On heating the material, negative particles, elec-
trons are driven out. Recall the action in an operating vacuum tube. A high speed parti-
cle on passing through a gas knocks highly mobile negative particles, electrons, out of
molecules in its path. This action is employed in a counter tube.
In 1911, Rutherford reported that substantial foils of material were largely open to
the free passage of alpha particles, high speed helium nuclei. But some would come near
positive centers and be deflected through large angles. The results were explained by
considering each atom to be composed of a small relatively massive positive nucleus sur-
rounded by an electron cloud.
So a molecule consists of an array of nuclei, surrounded by core electron clouds, held
together by the cloud of valence electrons. The nuclei of a two-atom molecule lie on a
straight line. When the two atoms are the same, the centers of positive and of negative
charge coincide and the molecule has no dipole moment. On the other hand, when they
are different, these centers do not coincide and the molecule exhibits some dipole
moment. In any case, we say there is a chemical bond between the two atoms. When this
involves two valence electrons, it is called a single bond. When it involves four valence
electrons, it is called a double bond. When it involves six valence electrons, it is called
a triple bond. Each of these has an equilibrium length.
When the valence of an atom is not satisfied by its bonding to a second atom, it may
bond to one or more additional atoms. Thus, many polyatomic structures exist. Some of
these are linear; others nonlinear. In any case, an equilibrium length can be assigned to
each bond. And, an equilibrium angle between any two bonds from a given atom. Fur-
thermore, a definite dipole moment is associated with each bond, directed from the more

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