A formula is the product of a compound placed out using element symbols in the necessary ration. The format used depends upon what the writer is trying to communicate - a standard molecular formula for water is H2O, for instance, but could be written as OH2 if the writer is holding it in a list of hydrides. The metal biotite could be rewritten as K(Mg, Fe)3(AlSi3O10)(F, OH)2, if the writer wants to highlight the different elements in the structure, but could also be written as KMgxFe3-xAlSi3O10+zHzF2-z if the writer wanted to concentrate on how much metal was in the structure.
A molecular model is usually a physical model of the structures in a compound. These may be produced by students . Infrequently, people relate to computer graphic images of molecules as molecular models.
