Space-filling models are three-dimensional representations of molecules that illustrate how atoms occupy space. These models use spheres to represent atoms, with the size of each sphere corresponding to the atom's radius. By connecting these spheres, space-filling models provide a visual understanding of molecular structure and how different atoms interact within a compound. These models are particularly useful in chemistry and biochemistry for visualizing complex molecules like proteins and DNA. They help scientists and students grasp concepts such as molecular geometry and steric hindrance, which are important for understanding chemical reactions and biological functions.