no remember if that is atom, you will discover it image in periodic table and that is for freed from cost. because of the fact by utilising defination, the kind of proton and electrons could be equivalent in extensive kind for an atom For ion, that is formed by utilising the addition of subtraction of electrons from or to an atom. hence, it ought to have cost (eg Ca^2+) For fomula unit, from the defination from wikipedia. that's the backside total extensive kind ratio of ions represented in an ionic compound. Strickly conversing, that is not "a sort of compound". as an occasion, in a crystal of salt, there is billion of Na+ and Cl- ions exist interior the crystal. whether, because of the fact the ratio of Na+ and Cl- is mostly a million:a million. hence, we write NaCl quite of permit say "Na10000Cl10000. For polyatomic ions. From the call, all of us comprehend that "poly" propose "extra beneficial than one". hence, that is a mix of two or extra atoms and contain cost. For molecules. by utilising defination, a moleules can exist by utilising its very own in ecosystem. it is not comparable to ions. For ions, a cation (eg Na+) could be exist provided that a counterion exist (eg Cl-). whether, for a oxygen molecules as an occasion (O2), it may exist by utilising its very own. A molecule commonly would not have cost. Ionic compounds commonly sort lacttice shape yet not molecules. whether, be careful that some atom could be a molecule additionally. For noble gasoline, for an occasion, argon (Ar) can exist in atomsphere in its atomic sort. hence that is an atom and molecule concurrently.