Potassium is by far the coolest element. It's been my favorite ever since high school when my mad scientist chemistry teacher let us drop chunks in water!
Technetium, Tc (Z = 43), is pretty darn cool. Surrounded by “stable”elements, this one has *only* radioactive isotopes. Want some? You’ll have to make it by bombarding its neighbor molybdenum or fishing it out of a nuclear reactor. As for chemistry, it forms oxidation states 0,1,2,3,4,5,6, AND 7 (though +2 is apparently a bit hard to come by). Its use in nuclear medicine as a diagnostic tool is incredibly important.
The coolest element is iron (Fe; Z=26):
(1) (1) It’s the most stable of elements limiting stellar nucleosynthesis up to Fe; elements after Fe are created by supernova nucleosynthesis.
(2) (2) It forms the core of the Earth (along with Ni) which resulted in Earth’s large magnetic field protecting life from extraterrestrial radiation.
(3) (3) It’s the basis for hemoglobin without which our cells would not get sufficient oxygen to live.
(4) (4) It forms large deposits (the banded iron formations or BIFs) which allowed for steel manufacture and the modern industrial revolution.
I think that oxygen is pretty cool. It forms peroxides, water, many acids (at least in part), and even part of air we breath. We are appromately 70% water. That much oxygen + the oxygen making up the phospholipids, sphingolipids, sugars in DNA, amino acids, and other biochemical molecules + the regular intake of oxygen to our blood-streams received via respiration = large part of us. Oxygen is the most abundant element in the earth crust. In ozonolysis, oxygen can be a destructive force. It can form ethers, esters (favorite lab experiment thus far! ), alcohols, epoxides (pretty cool looking), carboxylic acids, ect.
Silicon is pretty darn cool. It, and it's derivatives, have so many practical uses in everyday life...from electronics to construction to manufacturing to a wide array of medical uses. I can't imagine life without it!
Tin (stannum) is the coolest element by far. It's got something for everybody with its 10 isotopes. You don't like tin-118? That's fine; we got tin-128! Not a huge fan of tin-119? That's cool; how about some tin-116 for ya?
Well, obviously the coolest element is Gallium, it melts in your hand and is not that dangerous to handle, unlike Caesium...
With gallium you can do some clear tricks like the disappearing spoon in hot tea or the beating heart...hours of fun!!!!