These purple atoms are used to represent trigonal bipyramidal atoms such as dsp3 hybridized phosphorus and is for extending your existing set or for replacing lost atoms. Each pack contains 10 balls with five holes in each ball in a trigonal bipyramidal arrangement. Each atom is 23 mm diameter.
Why would you need these? You could use them to represent phosphorus in, say, phosphorus pentafluoride, PF5.
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.