Great reference
Not only has this helped me in my chemistry class, but I appreciate the ability to search for an element by name if you have trouble finding it on the table. I also like how they provided the images (when available) of the elements themselves, in addition to providing information about oxidation states, physical properties, and isotopes. You can also customize the view you have of the table itself (i.e. color-coding it by element type, state, origin, discovery date, or radioactivity, etc.) which I really enjoy because I can learn more about the elements, their significance, and their relationship to each other. You can also choose to view the name of an element in a different language, and theres even a Wikipedia link imbedded in the Properties Inspector that will take you to the Wikipedia page for that particular element.
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iElements Periodic Table