In an ecosystem, why can only one species fill an ecological niche? (site 2)

An ecological niche is the functional role of the species in an ecosystem. The functional roles includes how individuals of a species derive food, which kind of habitat they acquire for survival, mating seasons in which they bred. In an ecosystem, only one species fill an ecological niche this is because of competitive exclusion principle which says that two or more species having same ecological niche in the same ecosystem compete for common  resources in an ecosystem. The one which is more competing and strong derives the resources better than the less competing and weaker species. Hence, in the end of the competition only one species is left filling an ecological niche.  

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds