Fluorine(F2) is the most reactive element, known to react with even Xenon, Krypton, Gold, Platinum, etc.
Fluorine is the most reactive element.
If by vital you mean most reactive, then the metal francium, Fr, is the most reactive element.
Fluorine
The most reactive is fluorine.
Potassium is most reactive with the element Oxygen. However, it should be noted that it is most reactive with a non-element: Water.
No, the opposite of that.
Potassium is the most reactive element with oxygen, readily reacting to form potassium oxide.
Fluroine is the most reactive non-metal.
Elements are more reactive as you go down the periodic table and to the left.
Fluorine
It should be noted that francium is not the most reactive element, but is the most reactive metallic element. Francium has the lowest electronegativity and the greatest electropositivity; the chemistry of francium is practically unknown today.
Chlorine is the most reactive gas element. Chlorine is in the halogen family.