A few weeks ago, I presented at ChemEd2013 "Flipping with Chemistry Apps" http://bit.ly/17cLPYY. One app that I mentioned that I use in my HS chemistry class on the iPad is the app Building Atoms, Ions, and Isotopes HD Lite which can be found here https://itunes.apple.com/us/app/building-atoms-ions-sotopes/id437001161?mt=8. This app is free (The Lite versions only includes building atoms and atomic theory history) and is distributed by Carolina Biological Supply. With this app students must determine the correct number of subatomic particles that are needed for the particular element they are given. Then students can place their finger on the correct subatomic particle and literally drag and drop the correct number of protons, neutrons, and electrons that are needed onto the model. Another feature that is shown is that when the students drag the electrons onto the model they will automatically fill in the correct principal energy levels.Once this model is complete I have the students take screenshots of the model showing the placement of the electrons. Once a library of atoms have been collected, then I have the students lay the iPads on the floor (for the space needed) and we construct certain groups of the periodic table aligning them one below the other. For example, I would have the students using their screenshots, place the screenshot of H, Li, Na, K, Rb in the correct order. With this visual picture I would ask the students to look for commonalities and trends within the group. From this activity I have enjoyed watching as students make the connection between valence shell electrons and additional principal energy levels. This activity has also allowed me to introduce ions and how charges are determined much easier than in previous years.