These medium length white links come in a pack of 25 and are used normally to represent single bonds. This pack of links is for extending your existing set or for replacing lost links. Grey or purple links are available as well - you might these to distinguish a particular feature of a molecule, or perhaps bonds classified as dative, or perhaps you just prefer a change. The Molymod system is the original, unique, dual-scale system of high quality low-cost molecular models. These enormously popular sets are ideal for students but are also used by scientists all over the world.