This pack of red atoms are used typically to represent oxygen atoms and is for extending your existing set or for replacing lost atoms. Each pack contains 10 balls with four holes in each ball in a tetrahedral array. Each atom is 23 mm diameter. Why would I need these? You could use them to represent a tetrahedral oxygen atom in the hydroxonium cation with the fourth hole attached to an orbital shape (available separately), or a tetrahedral oxygen atom in an ether or alcohol with the third and fourth holes filled by orbital shapes (available separately). 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. We also sell spare packs of atoms in different shades and with differing number of holes with 1, 2, 3, 4, 5, and 6 holes elsewhere in our shop to represent different atoms and atom types.