Mendeleev periodic table was a useful tool because it enables scientists to

Although mendeleev's periodic table wasn't approved, it was useful for the scientists since;

He left gaps. This gives us an idea that there are still other elements that are yet to be discovered.

He predicted the properties of those elements which were yet to be discovered and thus,helped in discovery of other elements later on.
The properties were similar to those predicted by mendeleev

The arrangement are classified into groups, it was very useful in studying and remembering the properties of large number of elements

They are all decomposition reactions.