One of the most important questions to ask of a material is whether it is #"molecular"# or #"non-molecular"#. Molecular materials include all gases, water, sugars, most organic species etc. We can thus speak of the #"molecularity"# of a substance, and #"molecular"# substances tend to have intrinsically LOWER melting points, and boiling points than #"non-molecular"# substances.
Non-molecular materials include network solids, metals, polymers..........and of course the smallest part of a such a material is a formula unit. So let us take a simple example, #NaCl#, or #CaCO_3#, which materials are of course #"non-molecular"#, and consist of an infinite array of sodium ions, and chloride ions, or calcium ions, and carbonate ions........And the smallest part of this material is of course the NEUTRAL species, the formula unit, #NaCl# or #CaCO_3#.
#"Claro....?"#
