An element contains one type of atom. O₂ is therefore an element even though each molecule contains two atoms. A compound contains different atom types chemically combined in a fixed ratio. A mixture contains more than one substance together, so composition can vary.
Representations serve different purposes. An element symbol identifies an element; a chemical formula shows atom types and numbers; a 2D or 3D particle model can show arrangement but uses conventional colours, sizes and bonds. A formula does not automatically show the exact 3D shape.
The strong legacy lesson also connects to the development of atomic and periodic ideas. Mendeleev used recurring properties and relative atomic mass, left gaps and predicted missing elements; the modern periodic table is ordered by atomic number. Use this as evidence that representations and scientific models can improve over time, without drifting into senior atomic structure.
Mixtures include a uniform solution, a suspension whose larger particles may settle, and a colloid whose dispersed particles remain suspended for longer.