sentences:  Space-filling model
  
  In chemistry, a space-filling model, also known as a "calotte model", is a type of three-dimensional (3D) molecular model where the atoms are represented by spheres whose radii are proportional to the radii of the atoms and whose center-to-center distances are proportional to the distances between the atomic nuclei, all in the same scale.Atoms of different chemical elements are usually represented by spheres of different colors.