Here's why:

* Fixed Composition: A compound is formed by the chemical combination of two or more elements in a fixed ratio. This ratio is always the same, regardless of the source of the compound.

* Chemical Bonds: The elements in a compound are held together by chemical bonds, which dictate the exact arrangement and number of atoms in the molecule.

* Consistent Properties: Because the composition is fixed, compounds have consistent physical and chemical properties.

Example: Water (H₂O) always has two hydrogen atoms and one oxygen atom per molecule. This fixed ratio defines its properties, like boiling point and density.

Contrast with Mixtures: Mixtures, on the other hand, can have varying compositions. Their components are not chemically bonded, so the proportions can change.

Let me know if you'd like more examples!