These black atoms are often used to represent carbon atoms. They are 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 carbon in, say, ethene or benzene in conjunction with the Molymod pink and purple orbital shapes to represent π-bonding.
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.