All those listed on the periodic table.
by not forming a compound or mixture with another element
hydrogen is but not oxygen
Yes. All elements from 1-20 are representative elements as they belong to a representative group (i.e. they aren't a transition metal).
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...