Nova Elements is one of those showcase apps for the iPad. It has a great UI and is filled with fantastic photos
The showcase section of the app is an interactive periodic table that provides details on each of the elements. You can read about each element find out where it is found and how it is being used. There's also a build mode that lets you add electrons, neutrons and protons to build the atom that makes up the element. You can choose an auto-build that'll assemble the atom automatically. If enjoy a challenge, you can build it yourself. If you make a mistake, though, the atom will explode.
Besides the periodic table, Nova Elements also includes a link to buy the Hunting The Elements television program from NOVA. Another section includes the David Pogue's Essential Element's game that challenges you to drag atoms into place in order to build the element. These side sections are fun, but it is the periodic table that's the centerpiece of the app.
Nova Elements is a fantastic tool for students and science geeks alike. It is available for free from the iOS App Store and is compatible with the iPad.