All the models for methane are equally valid ways of representing the same thing; one kind of model may be preferred over another, depending on the level of detail at which you wish to visualize a molecule.
First, we see the model, a representation of lines connecting the center of each covalently-bonded atom.
A model represents bonds as lines and atoms as spheres, very similar to physical models that you may have used in organic chemistry, which give an impression of structure.
models represent the molecule as a composite of spheres, the radii of which have been chosen to approximate van der Waals contact distances. Spacefilling models are intended to portray overall molecular size and shape.
Surface models can be generated in various ways. a simple van der Waal surface, a smoother representation than the generalized spheres of a spacefilling model. Another common model is the solvent-accessible surface model, which is generated by rolling an imaginary ball with the diameter of a water molecule (1.4 Å) over the van der Waals radii used for spacefilling models. The solvent-accessible surface is smoother and larger, especially for proteins, because water cannot fit into narrow crevices or depressions in the surface of protein.
of the surface model.
The two carbons at the end of a carbon-carbon single bond are free to rotate about that bond. As this rotation occurs, the steric energy of the molecule changes as the result of torsional strain and van der Waals repulsions between groups attached to the two atoms. In cyclohexane, the bonds are not entirely free to rotate, but partial rotation about carbon-carbon bonds results in different conformations of the cyclohexane ring.
The is shown first.
Lastly, the twisted or .
spacefill. Click here for more information on cyclohexane's conformations.
We are now ready to extend our review of basic principles to glucose, which is an oxacyclohexane (one oxygen atom replacing a carbon).