The "structural unit" of a molecular substance is a true molecule. Here are the models of some true molecules. The ( really colourless) atoms are represented in conventional colours. (all at x1000000000 magnification !)
The "structural unit" of the noble gases (gaz rares, gaz nobles) is a monoatomic "molecule" (made of one single atom)
The atoms of metals and salts are structured to lattices (regular stgructures in space). Therefore one cannot make out molecules, but rather "structural units" reproduced regularly in space. In the case of the sodium chloride lattice, the structural unit consists of one sodium and one chlorine atom.
In the case of the iron lattice, the structural unit consists of one iron atom.
As not all substances are constituted of molecules, it is better to define a chemical substance as the set of all "structural units " of one kind