Carbon forms four bonds and can hybridize as sp3 (tetrahedral, 109.5°), sp2 (trigonal planar, 120°), or sp (linear, 180°), with s-character increasing in that order. Higher s-character means shorter, stronger, more electronegative bonds and more acidic C-H. IUPAC naming requires the longest chain containing the principal functional group, numbered to give that group the lowest locant, with substituents listed alphabetically as prefixes. Structural isomers share a molecular formula but differ in connectivity; stereoisomers share connectivity but differ in spatial arrangement. Geometrical (E/Z) isomerism requires both restricted rotation and different substituents on each carbon of the restricted bond. The inductive effect operates through sigma bonds — alkyl groups are +I (donate) while electronegative groups like -F, -Cl, -$NO_{2}$, and -COOH are -I (withdraw). The mesomeric effect operates through conjugated pi systems — -OH and -$NH_{2}$ are +M (donate lone pair), while -$NO_{2}$ and -CHO are -M (withdraw). Hyperconjugation is a no-bond resonance of alpha C-H sigma electrons into an adjacent empty p-orbital or pi bond, stabilizing carbocations, radicals, and alkenes. Carbocation stability follows 3° > 2° > 1° > $CH_{3}^{+}$, while carbanion stability is exactly reversed: $CH_{3}^{-}$ > 1° > 2° > 3°. The four organic reaction types — substitution, addition, elimination, and rearrangement — are defined by whether groups are replaced, added across a multiple bond, removed to form unsaturation, or internally reorganized.