Ionic charges and co valence numbers determine what empirical formu las are possible. The rules for writing Lewis structures of molecules. We need structures because the physical and chemical properties of molecular compounds are related to them. It gives us the geometry or shape of a molecule. At the molecular level of life the shape of a molecule is as important as other features of its structure.
Shape bears on a molecule's overall polarity.
When we know the relative electronegativities of the atoms at opposite ends of a covalent bond, we know how the bond is polarized. When we combine this information with the molecule's shape, we learn something about the overall polarity of the molecule. When we know this, we can make inferences about the substance's physical state and its solubility in various liquids. The vocabulary of acid-base neutralization reactions. In later chapters we'll be dealing with the dependence of our breathing suc cessfully on the acid-base balance of the blood.