In my high school chemistry class, a unit we cover is that of atomic structure. In particular, given an elements symbol, mass number, atomic number, and charge, the objective would be for the student to determine the atoms number of protons, neutrons, and electrons. I have several apps/program suggestions that can be useful for this purpose.
In my high school chemistry classes, I stress the use of units and the use of written chemical formulas to be represented properly. It is important to me that when a student expresses the formula of a chemical either in their data or in a balanced equation that they represent it correctly.
A few weeks ago, I presented at ChemEd 2013 "Flipping with Chemistry Apps". One app that I use in my HS chemistry class on the iPad is the app Building Atoms, Ions, and Isotopes HD Lite.