Q.What is a homologous series? Using the alkane series (general formula CnH2n+2) as your example, write the molecular formulas of the first four members and state how any two successive members differ.

With millions of known organic compounds, classifying them by two independent lenses -- overall carbon-skeleton structure, and functional group -- is what makes the subject navigable at all.

By structure, every compound is first split into acyclic (open-chain/aliphatic, like propane $CH_3-CH_2-CH_3$, no ring anywhere) or cyclic (containing at least one ring). Cyclic compounds split again by ring composition: homocyclic (the ring is built entirely of carbon) versus heterocyclic (the ring contains at least one non-carbon atom, commonly O, N or S); and each of those splits again by aromaticity. So a homocyclic ring is either alicyclic/carbocyclic (a plain, non-aromatic carbon ring, e.g. cyclopropane) or aromatic -- itself either benzenoid (built around an actual benzene ring, e.g. phenol) or non-benzenoid (aromatic without a benzene ring, e.g. azulene). A heterocyclic ring is either alicyclic (a non-aromatic heteroatom ring, e.g. tetrahydrofuran) or aromatic (e.g. pyridine). Applying the scheme in practice just means asking, in order: is there a ring at all? if so, does it contain a heteroatom? and is it aromatic?

By functional group, compounds are grouped by the reactive atom or group of atoms that gives the molecule its characteristic chemistry -- this is the classification the whole rest of this unit (and every later unit) actually leans on. The book's Table 11.1 catalogues roughly thirty classes: the unsaturated hydrocarbons (alkenes, $C_nH_{2n}$, alkynes, $C_nH_{2n-2}$, defined by their C=C or C≡C bond alone), the halide/oxygen/sulfur single-bonded families (alkyl halides R-X, alcohols R-OH, ethers R-O-R', thiols R-SH, thioethers R-S-R'), the carbonyl-based families sharing a C=O but differing in what else sits on that carbon (aldehydes R-CHO, ketones R-CO-R', carboxylic acids R-COOH, esters RCOOR', acid anhydrides R-CO-O-CO-R', acyl chlorides R-COX, amides R-CO-NH2), the nitrogen- and sulfur-oxide families (sulphonic acids R-SO3H, nitro compounds R-NO2, nitroso compounds R-NO), the amine/nitrile family and its structural cousins (amines R-NH2, nitriles R-CN, and the isomeric group of isocyanides R-NC, cyanates R-OCN, isocyanates R-NCO, thiocyanates R-SCN, isothiocyanates R-NCS), and a handful of nitrogen-nitrogen and other families (imines R-CH=NH, hydrazines R-NH-NH2, hydrazo compounds R-NH-NH-R, phenols C6H5OH, and imides RCON(R')COR'').