© 2015 Universidad Nacional Autónoma de México, Facultad de Química. This article reflects on the notion of context and its importance in the teaching and learning of science in general and chemistry in particular. A brief review of the various ways of understanding science education in context and problems associated with its application in the classroom is done. From this analysis a proposal of criteria for the development of contextualized learning units is presented, and its goal is to construct a model with the key ideas to be applied. As an example of this, the design of a teaching unit is shown for an atomic model that takes into account a possible scenario of progression in their learning.