Science Project! (Atomic Models)

Yesterday, on my Science class, Mrs. May (my teacher) told us to use "balls" and their "connectors" to make a certain element. I made CO2 or commonly known as Carbon Dioxide. It's really fun to connect them, especially when I tried making a  complex ones like C6H12O6 or Glucose. Okay, now I'll explain. The red balls represent Oxygen while the black ball represents Carbon. When two molecules of Oxygen and one molecules oh Carbon attached to each other, they'll make an element called Carbon Dioxide. But I still wonder, why each types of balls has a different numbers of holes??

                                                   (Photo by Fakhruni Alisya)