I'm always on the look out for a good PWA (Progressive Web Application) that runs well in Edge on a Windows 10 Mobile phone - and this is excellent. There's a slight caveat in that smaller screened phones may not manage the two-paned view in Periodex, but it's full of data in gory elemental detail.
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!