Groups of specialized cells that work together are called: organ systems
tissues
muscles
tendons

Groups of cells working together form tissues, groups of tissues working together form organs, organs working together form organ systems, organ systems cooperating and working together form organisms.

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