n-Octane. Molecular model of the hydrocarbon n- octane (formula: C8H18). The atoms are spherical and colour-coded: carbon (blue) and hydrogen (white). n-Octane is a chemical which is distilled from crude oil largely for use in petrol. The n refers to the straight chain form, in which all the carbons are arranged in an unbranched line, as seen here. It is one member of a family of 18 octanes, all of which have the formula C8H18. The octane number" of a fuel is a measure of how efficient it is compared to a standard. The higher the number, the more efficient the fuel."