Which is the only element that defies gravity?

Well according to this BBC article
http://www.bbc.co.uk/news/magazine-24903034
Helium is one of several.
"In the universe as a whole, it is one of the commonest elements, second only to hydrogen in its abundance. On Earth it is relatively rare, and one of the few elements that escapes gravity and leaks away into space."

I would point out that it doesn't really 'defy' gravity. It's just that we don't have enough gravity to control it. It's almost totally non-reactive, unlike the less dense hydrogen atom, and is found simply as He. The molecule is one atom, and a very 'light' one at that. Hydrogen is still around on Earth because it reacts with so many things (including oxygen, which can be lead to a nice explosion and some more water...). Most of the Helium that was here to start with has long gone, and what we have was formed by decay of radioactive things underground so it got trapped.
https://en.wikipedia.org/wiki/Helium