Final answer:
NH4Cl forms an ionic bond not because of the electronegativity difference between nitrogen and chlorine, but because it consists of the ammonium ion NH4+ and the chloride ion Cl- which attract each other to form a crystal lattice structure typical of ionic compounds.
Step-by-step explanation:
The compound NH4Cl forms ionic bonds despite there being a zero electronegativity difference between nitrogen and chlorine because it's comprised of two ions: NH4+ (ammonium) and Cl- (chloride). Instead of looking at the electronegativity difference between nitrogen and chlorine, it's necessary to consider the ammonium ion as a whole. The difference in electronegativity between hydrogen in NH4+ and chlorine in Cl- is not zero. Additionally, ionic bonds do not solely depend on the electronegativity difference but also on the ability of one atom to donate electrons and the ability of another to accept them, leading to the formation of ions which subsequently attract each other through electrostatic forces.
Although often associated with a large electronegativity difference, ionic bonding can also occur in compounds like NH4Cl due to the crystal lattice structure it forms, similar to other ionic compounds such as NaCl. The lattice encompasses positive (NH4+) and negative (Cl-) ions attracted to each other, stabilizing the structure. So, NH4Cl is an ionic compound because it results from the electrostatic attraction between cations (NH4+) and anions (Cl-), rather than from sharing electrons as in covalent bonding.