connect tha balls eith some of the sticks. (assemble a skeleton structure for the molecule,joinning atoms by single bonds)
in some cases this can only be done in one way.usually however,there are various posbysingle bonds some of which are reasonabla than others. in ch20 the model can be assembled by connecting the two H atom balls to the c atom ball with of the available sticks. and then using a third stick to connect the c atom and o atom balls.