The concept of the chemical bond as a direct link between atoms can be patterned by joining balls (atoms) with sticks/rods (bonds). This has been exceedingly popular and is still widely used today.

At first atoms were made of spherical wooden balls with specially drilled holes for rods. Hence carbon atom can be presented as a sphere with four holes at the tetrahedral angles cos-1(-1/3) ? 109.47°.

A trouble with fixed bonds and holes is that systems with arbitrary angles could not be built. This can be overcome with flexible bonds, originally helical springs but now usually plastic. This also allows double and triple bonds to be estimated by multiple single bonds, as shown in picture.

proline molecular model

Picture comprises a ball-and-stick model of proline

The balls have different colours: black represents carbon (C); red, oxygen (O); blue, nitrogen (N); and white, hydrogen (H). Each ball is drilled with as many holes as its conventional valence (C: 4; N: 3; O: 2; H: 1) directed towards the vertices of a tetrahedron.

Single bonds are represented by (fairly) rigid grey rods. Double and triple bonds use two longer flexible bonds which restrict rotation and support conventional cis/trans stereochemistry.