From previous lessons (see text) students should have an understanding that all matter consists of particles that adhere together in various ways to give solids, liquids, and gases. But what is the nature of the those particles (atoms)? The following video lecture provides an overview of the history of that discovery.
Note, that the development of our current understanding of atoms involved painstaking investigations of many scientists over a long period of time. If you Google: Mystery of Matter/ you will find a series of videos that are reenactments of the various scientists conducting their investigations. Note their primitive laboratories and equipment, which are pictured as well. Videos are a bit lengthy (30 minutes or so) but it is worth showing kids that our understanding of atoms did not always exist nor was it one simple discovery. And the search is still going on.