The Molymod 20mm grey covalent link is the most common bond & is used in virtually every model. Buy the 20mm grey links to augment or replace those in any Molymod product.
The MolyMod products are great. They are high-quality and easy-to-use. I use the models to help my biology major students understand the structures of biological macromolecules. The hydrogen bonds (purple links) and atoms with extra holes are extremely useful for showing base paring between nucleotides. The customer service I have received from Indigo Instruments over the years has been outstanding.