Question: Why is hydrogen so isolated on Mendeleev's Periodic Table? While it is a special case in terms of how its properties fits the pattern, surely there are other elements like it?

Edward Banks answered on 18 Mar 2020:

It really is unique! It’s the smallest element, having only one proton and electron. The properties of elements are usually defined by how they interact; this in turn is governed by how ‘full’ their outermost shell of electrons is.
Electrons in atoms are arranged in concentric ‘shells’ of varying size; the smallest shell can hold a max of 2 electrons, the second holds a max of 8 and so on. If an element has a full outer shell it doesn’t react very much, these are called the noble gases. If it has only 1 electron in the outer shell it reacts in certain ways, and if it has 1 less than the the max number for its outer shell it reacts in different ways.
Now because hydrogen’s outer shell can only hold 2 electrons and it already has 1; it has 1 more than the minimum and 1 less than the maximum; meaning it has properties of both groups!