Why does $\ce{NH4Cl}$ form an ionic bond if the difference in electronegativity between nitrogen and chlorine is $0$?
Shouldn't it be at least 1.7 in order for an ionic bond to form? If not what is the reason behind this?
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3$\begingroup$ Ionic bond bond is between ammonium cation and chloride anion, there's no bond between N and Cl. $\endgroup$– MithoronMay 14 at 16:16
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3$\begingroup$ I think you should rather be happy, that you got any answers, because the question is hardly outstanding. $\endgroup$– MithoronMay 14 at 17:08
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2$\begingroup$ Well, compare your post with Ananta's answer, it doesn't look remotely similar, doesn't it? And yes, questions can look like that, not only answers. If they did, upvotes would follow. $\endgroup$– MithoronMay 14 at 17:17
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3$\begingroup$ I recommend making the following edits: (1) Make sure it is error free; 'o' should be replaced with $\pu{0}$, '??' should be replaced with '?'. (2) Research and cite sources; why should we believe that the electronegativity difference between two atoms should be at least $\pu{1.7}$ to form an ionic bond? Of course, we cross-checked it, but knowing your sources is important. $\endgroup$– anantaMay 14 at 17:20
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3$\begingroup$ It looks to me you may consider NH4Cl similar to CH3Cl. But there is huge difference. There is direct covalent bond C-Cl, but no bond N-Cl. N atoms cannot form 5 bonds. $\endgroup$– PoutnikMay 14 at 19:35
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4 Answers
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Quick Answer
Explicit valence of $\ce{N}$ is four, $\ce{N}$ of $\ce{NH4+}$ cannot form another (covalent) bond with $\ce{Cl-}$.
The chemistry of $\ce{NH4+}$, owing to similar size, charge, and polarizing power, is similar to that of alkali metal ions, especially $\ce{K+}$; $\ce{NH4+Cl-}$ is ionic just like $\ce{K+Cl-}$ is, with about $\pu{80\%}$ ionic character.
The gap between HOMO of $\ce{Cl-}$ and LUMO of $\ce{NH4+}$ is about $\pu{5 eV}$ making any covalent interaction rather weak; $\ce{NH4+}$ doesn't have available empty orbitals to make a covalent bond with $\ce{Cl-}$.
The Details
Maximum Valency and Complete Octet of $\ce{NH4+}$
It is commonly known that $\ce{N}$ does not form more than four bonds. A similar question (How many bonds can nitrogen form?) on chemistry stack exchange answered this. Since, $\ce{N}$ in $\ce{NH4+}$ forms four bonds with four $H$, another covalent bond is not possible; moreover, it is forming a complete octet, which is especially stable for elements with atomic number $Z \leq 10$.
Similarity between $\ce{NH4+}$ and Alkali Metal Ions
$\ce{NH4+}$, due to its size and charge, as you mentioned, often shows similarity to alkali metal ions, especially $\ce{K+}$. By this analogy, since alkali metals for ionic bonds with halides, so does the $\ce{NH4^+}$ cation.
According to reference$^1$
(Thus,) the room-temperature $\ce{NH4+Cl-}$ bond length estimated from equation (5) is $\pu{3.28} \overset{\circ}{\text{A}}$ , corresponding to the $r_\ce{NH4+}$ value of $\pu{1.47} \overset{\circ}{\text{A}}$.
Comparing with values given in reference$^2$, this value falls within the (Pauling) ionic radii of $\ce{K+}$ ($\pu{1.33} \overset{\circ}{\text{A}}$) and $\ce{Rb+}$ ($\pu{1.48} \overset{\circ}{\text{A}}$). Also note the Gourary and Adrian Electron-density (ionic) radius of $\ce{K+}$ is $\pu{1.49} \overset{\circ}{\text{A}}$.$^2$. Thus, $r_\text{ionic}(\ce{NH4+}) \approx r_\text{ionic}(\ce{K+}), r_\text{ionic}(\ce{Rb+})$.
Especially in Biochemistry, chemistry of $\ce{NH4+}$ has often been compared with that of $\ce{K+}$. For example, in reference$^3$, it is stated:
(We conclude) that a unique transporter, a potassium-ammonium ($\ce{K+}$/$\ce{NH4+}$) antiport, is responsible for $\ce{NH4+}$ transport in renal inner medullary collecting duct cells. This antiporter is sensitive to verapamil and Schering 28080, is electroneutral, and is selective for $\ce{NH4+}$ and $\ce{K+}$ as substrates. The $\ce{K+}$/$\ce{NH4+}$ antiporter may play a significant role in acid-base regulation by excretion of ammonium and elimination of acid.
Citing reference$^3$, it is stated in reference$^4$
Since host–guest chemistry may also include non-metallic cations, it is interesting to note that the K+ ion has been stated to be similar to the ammonium ion ($\ce{NH4+}$).
Large gap between HOMO of $\ce{Cl-}$ and LUMO of $\ce{NH4+}$
Furthermore, as calculated by MolCalc, the difference between the HOMO of $\ce{Cl-}$ and LUMO of $\ce{NH4+}$, both lying at energies greater than zero ( $\pu{2.73 eV}$ for $\ce{Cl-}$ and $\pu{7.21 eV}$ for $\ce{NH4+}$), is about $\pu{5 eV}$, making any covalent interaction rather weak (not occurring in this case).
References
On the effective ionic radii for ammonium. Sidey, V. (2016). Acta Cryst. B72, 626-633. 10.1107/S2052520616008064
Conway, B.E., Ayranci, E. Effective Ionic Radii and Hydration Volumes for Evaluation of Solution Properties and Ionic Adsorption. Journal of Solution Chemistry 28, 163–192 (1999). 10.1023/A:1021702230117
Hassane Amlal, Manoocher Soleimani, K+/NH4+ antiporter: a unique ammonium carrying transporter in the kidney inner medulla, Biochimica et Biophysica Acta (BBA) - Biomembranes, Volume 1323, Issue 2, 1997, Pages 319-333, ISSN 0005-2736, 10.1016/S0005-2736(96)00200-3
Kim, Y., Nguyen, TT.T., Churchill, D.G. (2016). Bioinorganic Chemistry of the Alkali Metal Ions. In: Sigel, A., Sigel, H., Sigel, R. (eds) The Alkali Metal Ions: Their Role for Life. Metal Ions in Life Sciences, vol 16. Springer, Cham. 10.1007/978-3-319-21756-7_1
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3$\begingroup$ +1, very nice answer, but perhaps "a cannon for sparrows". $\endgroup$– PoutnikMay 14 at 17:01
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3$\begingroup$ @Poutnik I do try to write as succinctly as possible, but the more I read, the more curious I become, and the more I read on, and I feel sharing knowledge is essential. Perhaps this is my writing style. $\endgroup$– anantaMay 14 at 17:08
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As there is no bond between nitrogen and chlorine in ammonium chloride, it cannot be either ionic either covalent. Nitrogen bonding in an ammonium cation is fully saturated by four covalent bonds to four hydrogen atoms.
It has the same symmetry but with opposite charges as potassium perchlorate. There is no bond between potassium and chlorine either.
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$\begingroup$ Ionic bonds don't have to be bonds. Theoretically, a 100% ionic bond is merely an electrostatic attraction without any orbital overlap (or bond). $\endgroup$– anantaMay 14 at 17:10
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$\begingroup$ @ananta I do agree. Even practically. I mentioned it in context of evaluation of ionic percentage of eventual mentioned N-Cl bond. $\endgroup$– PoutnikMay 14 at 17:15
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$\begingroup$ @ananta I'd rather say chemical bond needs something more than just electrostatics. That's true for hydrogen bonds, agostic interactions, etc. Related chemistry.stackexchange.com/questions/86918/… $\endgroup$– MithoronMay 14 at 19:10
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$\begingroup$ @Mithoron I agree. I only meant a theoretical (perhaps hypothetical would have been a better word) $\pu{100 \%}$ ionic bond would mean only electrostatic attraction or electrostatic potential minimization. Since this is not possible, at least in chemistry, and perhaps even in physics (because, otherwise, opposite charges would come infinitesimally close to each other, which doesn't happen), there is always some interaction that cannot be attributed to electrostatics alone. $\endgroup$– anantaMay 14 at 19:18
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$\begingroup$ @Mithoron It depends on the point of view. All chemical bonds are merely just electrostatics, somewhat complicated by quantum mechanics phenomena. So if 2 or many ions are bound by their net charges, it is as the specific kind of electrostatic attraction a specific kind of a chemical bond. $\endgroup$– PoutnikMay 15 at 10:48
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Think of it this way. Electronegativity is defined as the tendency of an atom to pull electrons towards itself. So, say you have an atom having a positive charge, it will obviously having more power to pull electrons towards itself.
Having a difference of 1.7 is not something applying everywhere. There are ionic compounds with difference less than that.
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This value defined to $1.7$ is an order of magnitude, valid for about $50$% of all the bonds. It does not prevent ionic bonds existing between $\ce{N}$ (in the center of $\ce{NH4^+}$) and $\ce{Cl^-}$.
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$\begingroup$ Does N and Cl always form an ionic bond? Or is this a particular case? $\endgroup$ May 14 at 14:58
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1$\begingroup$ @Neagu Cristian. No ! N and Cl can make a molecule called $\ce{NCl3}$ which is made of 3 covalences $\ce{N-Cl}$. $\endgroup$– MauriceMay 14 at 16:31