Here's why:

* Elements: The simplest forms of matter that cannot be broken down into simpler substances by ordinary chemical means. Examples: oxygen (O), gold (Au), carbon (C).

* Compounds: Substances formed by the chemical combination of two or more different elements in a fixed ratio. Examples: water (H₂O), salt (NaCl), sugar (C₁₂H₂₂O₁₁).

Both elements and compounds are considered pure substances because they have a fixed and definite composition. They don't contain impurities or other substances.