Periodic anniversary

It is once again the anniversary of the Periodic Table of the Elements. It was first introduced to the world on this day in 1869, in a presentation to the Russian Chemical Society by Chemist Dimitri Mendeleev.

I write this with a sleeping baby in one arm, so it'll be brief.

First, a quiz for you - a fun activity for Periodic Table Day. Feel free to give your answers and other thoughts in the comments on this post.

1. What was periodic about Mendeleev's table?
2. What was the reason (discovered much later) for that periodicity?
3. How are the elements ordered in the table?
4. Where does hydrogen belong - on the left or on the right? Why?
5. What predictions did the original table suggest? Have they been confirmed?
6. Does the Periodic Table depict a fact or a theory? Explain.

Okay, now a bit of periodic fun. And another: [Sorry, link to final image seems to have gone dead.]