molecular structure — the arrangement of the atoms in a molecule. The molecular
structure is different from the molecular
formula in that the latter only tells what kinds of atoms are in a molecule and how
many of each kind of atom. The molecular structure tells us how those atoms are arranged in
space. Two molecules with the same molecular formula can have different molecular structures,
and therefore different properties. For example, salicylic acid and para-hydroxybenzoic
acid have the same molecular formula,
C7H6O3,
but they have slightly different molecular structures:
salicylic acid
para-hydroxybenzoic acid
So, the salicylic acid and para-hydroxybenzoic acid have very different properties.
Salicylic acid melts at around 158°C, while para-hydroxybenzoic acid melts at around
213°C. Aqueous solutions of salicylic acid turn purple in the presence of iron (III) chloride,
but aqueous para-hydroxybenzoic acid solutions do not.
What does all this mean? It means that since molecular structure determines properties, we can
change the properties of a material by changing the molecular structure of its molecules. This
is how chemists design new materials with the properties they want.
