This lecture covers the concept of isomers, molecules with the same molecular formula but different structural formulas, including constitution isomers and stereoisomers. It also explains the distinction between different compounds and isomers, as well as the types of stereoisomers like configuration isomers and conformers. The lecture delves into the conformation of alkanes, stereochemistry, and nomenclature of geometric isomers using the Cahn-Ingold-Prelog rules. It further discusses stereocenters, chirality, and molecules with multiple stereocenters, highlighting the nomenclature of stereocenters and the importance of understanding stereochemical notation.