In this system a variety of moulded plastic units indicate atoms or groups of atoms; consequently, the versatility of the traditional ball and spoke type models has been sacrificed to produce accurate units which can be rapidly, and accurately assembled into high molecular weight molecules which require a minimum of supporting rods. The units are precision moulded in individual colours according to the functional group or moieties represented. Apart from hydrogen each atom is represented by a sphere whose radius is one third the van der Waal's radius. Thus the full van der Waal's radius is given by the outermost edge of a similar sphere placed or imagined in contact with the first. The models therefore combine many of the attributes of the skeletal with those of the space filling type.

The connection between different units is by means of integrally moulded arms and no separate connecting links etc. are therefore required: the units are joined simply by pushing the arm of one unit into the socket of the next. Two nodules on each arm lock with an internal groove in each socket to prevent the accidental separation of units. The interatomic distances are automatically set by the depth of socket in each unit. It follows, therefore, that the interatomic distance is a property of each type of unit. Those containing a socket and arms may be joined in a linear or branched fashion as required. High density polyethylene is employed for backbone structures where greater rigidity is required, whereas for the side chains low density is more suitable.

Although chiefly designed for the construction of large molecules the units can be made in exact or close approximation to the structures of small molecules where the interest of the biochemist and organic chemist requires it. Occasionally, it is necessary to modify existing units to represent a chemically modified group e.g. the addition of a phenyl ring to an imidazole. In cases such as these, the units can be drilled or pinned to receive the additional group. The units are easily cut with a sharp blade. Bond angles may be changed for particular applications. It is possible to distort the units permanently, by heating, bending and then freezing in cold water .
H-BONDS
For protein structures hydrogen bonds may be formed by pushing silver steel wire into the units, and covering with 2mm diameter red plastic sleeving if desired. Alternatively, 4.5mm diameter red PVC sleeving can be slid over the H-arm and the arm of a special H-bond cap which clips over the O atom. This is useful when the models are to be taken apart and reassembled. For nucleic acids where the inter-base H-bonds are load bearing, stainless steel rod is used.
VAN DER WAALS RADIUS
TThe values assumed for van der Waal's radii are as follows: (A) Carbon tetrahedral, 1.50; Carbon aromatic, 1.70: Carbon amide, 1.45: Nitrogen tetrahedral, 1.50; Nitrogen aromatic, 1.70; Nitrogen amide, 1.45; Oxygen digonal 1.35; Oxygen keto, 1.35, Sulphur digonal, 1.70; Phosphorus, 1.50; Hydrogen is not represented by a sphere but the position of the centre of a H atom is at the end of each short arm. Where the arm is extended to form a connecting arm, which is not required, it must be cut back to the start of the square section with a sharp blade or side cutting nippers, to indicate hydrogen.