* Compounds are formed when two or more different elements chemically combine in a fixed ratio.

Let's analyze each option:

* 8H2: This represents 8 molecules of hydrogen gas (H2). Hydrogen gas is a diatomic element, meaning it exists naturally as two hydrogen atoms bonded together. It's not a compound because it consists of only one type of element.

* H2O: This is water, a compound formed by the chemical combination of hydrogen (H) and oxygen (O) in a 2:1 ratio.

* 6CaO: This represents six molecules of calcium oxide (CaO), a compound formed by the chemical combination of calcium (Ca) and oxygen (O) in a 1:1 ratio.

* 5HCl: This represents five molecules of hydrochloric acid (HCl), a compound formed by the chemical combination of hydrogen (H) and chlorine (Cl) in a 1:1 ratio.

Therefore, 8H2 is the only option that is not a compound.