What are the ferocious elements?

All those listed on the periodic table.

hydrogen is but not oxygen

Yes, the properties of the elements within a period vary from element to element. For example, the metallic character decreases and the non-metallic character increases from left to right within a...

At room temperature, there are 11 gases, and they are hydrogen, nitrogen, oxygen, fluorine, chlorine, and all 6 inert gases, helium, neon, argon, krypton, xenon and radon. It is mercury and bromine...

Yes. All elements from 1-20 are representative elements as they belong to a representative group (i.e. they aren't a transition metal).