Note that nitrogen is a diatomic molecule, so that before a nitrogen atom can bond with another atom the N-N bond must be broken. This happens at high temperatures, but at room temperature, it takes something special like a catalyst to break the N-N bond and allow a new compound to form. Iron gives electrons to oxygen. Thus it will not react with oxygen under ordinary conditions. Aluminum oxide forms a protective coating on the aluminum, preventing further oxidation.
Ionic compounds have one and only one formula.
Crude oil is a mixture of many different hydrocarbons of different size chains. In addition to all the different hydrocarbon molecules already present in crude oil, adding just one other atom such as O or Cl to hydrocarbon makes a totally different molecule. Thus it is possible to make literally thousands of different molecules from the hydrocarbons present in crude oil.