Ball-and-stick models and space-filling models (also known as Calotte models) are 3D or spatial molecular models which serve to display the structure of chemical products and substances or biomolecules.
With ball-and-stick models, the centers of the atoms are connected by straight lines which represent the covalent bonds. Double and triple bonds are often represented by springs, which form curved connections between the balls. The bond angles and bond lengths reflect the actual relationships, while the space occupied by the atoms is either not represented at all, or only denoted essentially by the relative sizes of the spheres.
The advantage of the ball-and-stick model as opposed to the Calotte model is that it allows better insight into the molecule’s bond structure.
Although no official standard seems to exist, model kits commonly use the CPK coloring.
In 1865, German chemist August Wilhelm von Hofmann was the first to make stick-and-ball molecular models, which he used in lecture at the Royal Institution of Great Britain, such as methane shown below: