RDKit is designed to be somewhat modular. It's a very very big package, and importing everything under Chem is not usually advised. The best practice when using the library is to be specific with your imports (use the Descriptors.ExactMolWt function instead of Chem.Descriptors.ExactMolWt). Alternatively, you can do from rdkit.Chem import AllChem which will load in almost everything in Chem module and its sub modules.