From the (super short) Appscope listing:
Elegant periodic table of elements
The URL to tap on or type in is periodex.co
Of course, you can turn this into an 'app' yourself with PAWA or just pin it 'as is' on your Start screen. Or just add it as a 'Favourite'. It's up to you. If you're confused by the different PWA options then check out my feature here.
Here's the PWA in action:
In portrait mode, even on the Lumia 950 XL, there's a little side scrolling needed to see the whole periodic table, and tapping on any symbol shows a scrollable pane with all its properties. However, even better...
... in landscape mode, with the navigation bar swiped away, Periodex works in full two-paned mode, so you can tap and swipe and generally navigate the interface with no page reloading. Super cool!