Cost: Part of CatPack App Pack
The Elements by Theodore Gray is a visual periodic table that allows you to find all the needed information on each element as well as a visual representation that gives you a greater appreciation for the building blocks of all matter.
The Elements in Action by Theodore Gray is a companion app that contains 79 video explorations of the properties of the elements. This functions as a standalone app but is even better when it is used in conjunction with The Elements.
Online Help and Tutorials
Brief Look at the Features of The Elements and The Elements in Action
Basic Guide to the tools and how to use Notability by the creator of the app, Ginger Labs
Video Overview of The Elements
This Lynda.com video gives an excellent overview of the app and what you can find using it.