Keep away from wrong kind of mastery of chemistry

Mastery-based learning
There are two meanings for "mastery" in Chemistry. 
Meaning 1 is "good skills and efficiency in manipulating its formal and symbolic apparatus". This is a sort of representation without the represented things (the objectivated concepts). For example, students say some agency is due to "elements" instead of "atoms". 
Meaning 2 is awareness of how the real world of atoms and substances actually are, how their behaviors are related. This awareness is what give us true appliable competencies, and the first kind of mastery is absolutely secondary in respect to the second. Khan Academy, one of the best guides to mastery learning on the web, can only affect the first kind of mastery, a kind that can be useless out of school life, cannot change the objectivating nature of concepts of learners. To achieve this result you need a true teacher who possesses that kind of awareness (of the same kind that researchers in chemistry must have), who foremost wants it to be transferred to the students, who has created strategies to do the right kind of mastery.