Question

whatz difference between "formula unit" and "molecule" ?

Answer 1

my notes has :

One mole of a covalent compound contains:
one mole (6.022 × 1023) of individual molecules.

One mole of an ionic compound contains:
one mole of formula units, bonded together.

Answer 2

"formula unit" is usually reserved for ionic compounds rather than molecules, since at an atomic level, ionic compounds like sodium chloride look like this: every positive ion is attracted to, and held onto, by up to 6 other ions, in an almost endless pattern that extends in all 3 directions. There's no difference which ion is "bonded" to which other ion, they are all held equally. A "formula unit" of sodium chloride is NaCl, but covalent compounds exist as discrete, separate units. Carbon dioxide molecules are separate from each other, and a pile of CO2 molecules doesn't exert (much) attraction to their neighbors.