These yellow atoms are used to represent, typically, sulfur (sulphur) atoms and is for extending your existing set or for replacing lost atoms. Each pack contains 10 balls with six holes at 90°. Each atom is 23 mm diameter.
Why would you need these? You could use them to represent octahedral sulfur in, say, sulfur hexafluoride, SF6.
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.