Molecules span a wide range of sizes. The smallest molecules contain only two atoms, and these diatomic molecules have length scales that are approximately the sum of the atomic radii of the constituent atoms. The smallest molecule, H–H, has a length of only 0.74 Ångströms (7.4 × 10⁻¹¹ m). Larger molecules can be comparatively quite large. Biologically important molecules, like proteins, often contain thousands of atoms. Polymers, which are highly linked networks of covalently bonded atoms, can be even larger still, sometimes becoming so large they are visible to the naked eye.