Answer is: PbO.
Formula unit is the representative particle of an ionic compound. It is lowest whole number ratio of ions in an ionic compound, for lead(II) oxide that is PbO.
Lead(II) is oxide with ionic bonds. Lead(II) contains one lead cations (charge of ion is 2+) and one oxygen aniona (charge for ion is 2-).
Other examples do not have ionic bonds.
Ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).
