A space-filling model is a three-dimensional representation of molecules that illustrates how atoms occupy space. This model uses spheres to represent atoms, with their sizes scaled according to their atomic radii. The spheres are arranged to show how the atoms are bonded and how they interact with each other in a molecule. These models are particularly useful in chemistry and biochemistry for visualizing complex structures, such as proteins and DNA. By providing a clearer picture of molecular geometry, space-filling models help scientists understand properties like reactivity and stability, which are crucial for fields like drug design and materials science.