Technetium is totally out of place. It sits smack in the middle of the most solid group of elements around, the transition metals. Virtually all the transition metals are nice stable metals, solid citizens of the periodic table. And here technetium is
radioactive! You've got to go a very long way up the list of atomic numbers before you run into another radioactive one (all elements above 83 are radioactive, but below that only 61 promethium and 43 technetium have no stable isotopes).
Why technetium is radioactive has to do with the way that protons and neutrons fit together in the nucleus. It's reasonably well understood why technetium can't find a stable configuration, and I think it's fair to say that it boils down to rotten luck. Here's an article about it.
Technetium is used for medical imaging, though this application is less popular now than it used to be. It can also be used to inhibit corrosion in steel, though of course it makes the steel radioactive.