Abstract
The experience below mentioned is a proposal to make the concept of molecule in 3º ESO clearer. The methodology is active, as it is directed to students who will follow a real process of investigation and will try to solve unknown problems. After the activity the chemical formulation appears as a necessity to represent the molecular structures in an easy and short way. A full description of the experience can be found on the internet,[1] where teachers can watch a video recorded during a class [2] and unload the proposed documents for investigation