When we speak of an element being reactive, we're generally talking about that element's desire or willingness to loan out or borrow electrons. You'll recall that the loaning and borrowing (along with the sharing) of electrons is what chemical bonding is all about. In chemistry, those atoms with the strongest desire to either loan out or borrow electons are the most reactive. And we find those atoms at the extreme left and extreme right of the Periodic Table (with the Group 18 elements, the inert or noble gases, excepted).
Fluorine is the most reactive element.
The element Californium is reactive.
Hydrogen is not only the lightest element of all, it is also very reactive.
Fluorine(F2) is the most reactive element, known to react with even Xenon, Krypton, Gold, Platinum, etc.
An element is said to be chemically reactive when it combines with another element. These two elements will create a new compound.
If by vital you mean most reactive, then the metal francium, Fr, is the most reactive element.
Potassium is most reactive with the element Oxygen. However, it should be noted that it is most reactive with a non-element: Water.
copper is very un-reactive. lead is more reactive.
Fluorine
Yes. it is.
XENON
I dont think i am sure about that