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Is an ionic compound like NaCl considered a dipole? It has a positive side (Na+) and a negative side (Cl-). Or is it true that an ionic bond does not have a dipole because a dipole is, by definition, a polar molecule?

Some people said it has a dipole, because the word dipole describes a separation of charge, so we could call an ionic compound a dipole.

While others said it doesn't have a dipole, because ionic compounds are held tight in a giant lattice, so the electrostatic forces of attraction are in all directions between all the ions. Thus, ionic lattices are not polar.

Does it maybe depend on the state of the ionic compound, whether it's solid, liquid or gas?

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    $\begingroup$ It must have a dipole because the di means two. So Na+ and Cl- must make up a dipole. $\endgroup$ Jun 25, 2023 at 17:38
  • $\begingroup$ So all ionic compounds are dipoles whether they're a solid, liquid or gas? And all ionic lattices are polar? $\endgroup$ Jun 25, 2023 at 18:06
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    $\begingroup$ Your question only makes sense for gas phase - there are discrete NaCl molecules in there, even if ionic component of bonding is large. $\endgroup$
    – Mithoron
    Jun 25, 2023 at 18:32
  • $\begingroup$ So, only in gas phase ionic compounds are considered dipoles (they're polar) because they exist as discrete molecules, but when they're in liquid or solid phase, they're not dipoles (they're nonpolar) because they don't exist as discrete molecules? $\endgroup$ Jun 25, 2023 at 19:08
  • $\begingroup$ In molten or solid salt... you could calculate dipoles for each neighbouring Na and Cl, but they all compensate each other - if you consider all neighbours you get zero. Also word "polar" is only good for solvents. Calling molecules like that is a dangerous mental short-cut, that leads to misconceptions. $\endgroup$
    – Mithoron
    Jun 25, 2023 at 21:39

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"A dipole" strongly implies a single thing having two poles. A molecule, like for example HCl, is a single thing. It consists of two atoms sharing a privileged spatial relationship, that stay close to one another (and much closer than to any other atom) for as long as the molecule exists. Because the shared electrons holding them together are unevenly distributed, the molecule has a "plus" and a "minus" side and is called a dipole.

It is a common mistake to see an ionic compound like NaCl and imagine it also contains some sort of fundamental entity, a distinct "molecule" consisting of a single Na and Cl atom and analogous to the real HCl molecule but held together by an ionic rather than covalent bond. The only place you'd see something like this would be in the gas phase, where ion pairs occur in rapid equilibrium with dimers (Na2Cl2) and larger clusters. For the time they exist under conditions you'll probably never encounter, these ion pair could be considered molecules and they are obviously highly polar, so yes, they would be dipoles.

Under ambient conditions though, ionic compounds are usually solids, in which each positive ion is surrounded by many more negative ions than the one (in the case of NaCl) it's "supposed" to bond with according to its empirical formula. Moreover, these opposite charges are, by defintion, symmetrically distributed around the ion (if they weren't, the lattice wouldn't be in equilibrium and would shift until it was). Each individual ion in the bulk of an ionic solid is in a perfectly apolar environment, so it doesn't make sense to talk about dipoles here.

The same logic holds for molten or dissolved salts, just that the ions move about randomly in stead of being stuck in a lattice; they're still doing that as individual ions, not ion pairs, so again there is no single thing to be called "a dipole".

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    $\begingroup$ -1 because there's no free ions in gas phase chemistry.stackexchange.com/questions/2511/… chemistry.stackexchange.com/questions/14174/… $\endgroup$
    – Mithoron
    Jun 26, 2023 at 14:15
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    $\begingroup$ Not all crystals containing ions have charges that balance out: ferroelectric crystals exist where ions can be polarised the same way magnetic domains can be polarised in ferromagnetic solids, giving the whole crystal a net dipole moment. $\endgroup$
    – matt_black
    Jun 26, 2023 at 19:42
  • $\begingroup$ @Mithoron You are right and I have edited my answer to reflect this. $\endgroup$ Jun 29, 2023 at 19:55
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Context is important for any coherent answer

The question is more subtle than it looks because the context matters for the answer.

Does an ionic bond have dipole? Yes, by definition: some separation of charge must create a dipole. But does a solid substance, NaCl, have any net dipole: no, as all the constituent dipoles cancel out when averaged over the whole crystal. And, in a typical ionic crystal, it isn't even clear that the idea of a single ionic "bond" makes sense as the bonds are not discrete entities as they would be in a molecular solid.

So, if the context of the question is whether the interaction between single ions of chlorine and sodium creates a dipole, the answer is "yes". But if the question is whether the bulk solid has a dipole, the answer is no. If the question is whether discrete gas-phase units with dipoles exist in NaCl vapour, the answer is maybe (depending on conditions).

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As a supplementary answer, the common definition of a dipole is a separation of charges, this separation means different the presence of opposite charge particules or quasiparticles. To be clear a proton (positive charge) and an electron (negative charge) should normally form a dipole (two poles) but the quantum ground state of this system is not a dipole, the elwcrronic distribution is symmetric and there is no dipole moment. Therefore, a dipole should definitely have a dipole moment.

In the case of the polar molecules as free NaCl, there is a dipole moment, so it is a dipole. However the crystal form of NaCl do not have a dipole moment and is not a dipole : the dipole moments of pairs cancel each other. There is no need of a pair of two elements to form a dipole (HCl) even larger molecules (donor-sigma-acceptor) form a dipole.

A set of polarized dipoles is almost always an unstable configuration, they always interact to form a gas, solid but especially liquid etc. where the total dipole moment is zero. The same configuration appears for magnetic dipoles by forming magnetic domains with a zero total magnetization.

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A simple explanation can be that a dipole occurs for a molecule. Keeping this in mind we can say that until and unless the ionic compound maintains a molecular identity, it will have a dipole. This is possible only if the ionic compound exists in a gaseous state; since in solid state it forms a lattice structure where individual molecules are indistinguishable and in molten or aqueous form, the ions dissociate into individual ions.

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From the Merriam-Webster dictionary definition of dipole:

ˈdī-ˌpōl

1 a : a pair of equal and opposite electric charges or magnetic poles of opposite sign separated especially by a small distance

b: a body or system (such as a molecule) having such charges or poles

2: a radio antenna consisting of two horizontal rods in line with each other with their ends slightly separated

Thus, there is ionic bond, like in the mentioned molecular $\ce{NaCl}$, as from definition 1a from that dictionary.

As to the question of whether an ionic bond has a dipole or not, from the dictionary definition b for dipole, the body or system needs to have such charges or poles. The reason that this can be a little tricky to understand is that a system of bonds together may not contribute any dipole moment, when the individual plus dipoles and minus dipoles of the bonds in the system cancel out, given a net zero dipole moment for the overall molecule.

So it is important then to reference if a, one is describing a specific bond of equal and opposite charges or if b, one is considering a system containing multiple charges, perhaps equal and opposite or perhaps not.

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Well the thing is dipole only considered in the case of covalent bond because in covalent bond, the bond is formed by mutual sharing of electrons between two atoms and in which the more electronegative atom attracts the shared pair of electrons known as electronegativity of that element that creates a dipole moment or polarity in a bond but in case of ionic bond there is no such sharing of electrons, instead there is a complete transfer of electrons for an example in NaCl the electron is transferred from sodium to chlorine which cause the formation of cation and anion now there is no sharing but there is a complete transfer and due to these formation of cation and anion they got charges and according to the law opposite charges attract each other and eventually electrostatic bond formed now Fajan's rule comes in action as he suggested that if a cation has a really powerful nucleus that means it has a high polarising power which attract the electron cloud of anion toward itself and cause a little polarity which induce a little covalent character in ionic compounds for an example LiCl is more covalent than NaCl due to the more powerful nucleus of lithium which causes high polarization power to the electron cloud of chlorine. (Feel free to correct me!!!)

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