Chemistry

yes, most substances exist as compounds(which are composed of the elements in the periodic table. e.g. water(composed of hydrogen and oxygen), rust(composed MAINLY iron and oxygen)

Only some substances exsist as elements, e.g. diamond, which is carbon, iron, aluminium...........etc.

thank you!

neutron stars are not composed of any elements. they're merely closely-packed neutrons.

What do you mean by "things"?