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I have asked a question loosely asking this, where I confused terms and did not specify what I wanted to know here, so I formed a new question.

What are the differences and similarities between ionic bonds and covalent bonds and ionic bonds and dipole-dipole interactions?

It seems to be that an ionic bond is nothing but a stronger dipole-dipole interaction. The only difference is where the electrons are.

For example, let's take a sodium cation and chloride anion bond. The sodium had its electron taken away from it. It is now positive. Chloride had an electron added to it and it's feeling negative. Now the two will attract because of these charges.

Now let's take two water molecules. The hydrogen of one will be positive because it is less electronegative and thus the electron density will be greater around the oxygen, which is more electronegative, making it negative. Now the hydrogen of one and the oxygen of another will attract due to the charges.

It seems like the only difference is that if you split the sodium chloride and the water's hydrogens and oxygen, the sodium and chloride will still have a charge, but the hydrogens and oxygen will not.

By textbook, a dipole-dipole interaction is an intermolecular force while a covalent bond, as well as an ionic bond, is a chemical bond. It seems to me that, yes, the forming of ions is a chemical reaction, but the ionic bond is an intermolecular force.

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    $\begingroup$ Covalent bonds are directional but ionic bonds are not. A cation can be surrounded by a number of anions and vice versa and they will have ionic bonds equally strong. $\endgroup$ – Papul Jun 7 '15 at 16:47
  • $\begingroup$ That sounds to me like the ionic bond is even more like a dipole-dipole interaction. $\endgroup$ – Dylan Spano Jun 7 '15 at 17:14
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An ionic bond could maybe be described as an inter-ionic force. All electron interactions are most accurately described by wavefunctions and quantum mechanics, but in practice we use successively more detailed approximations for convenience, stopping at the lowest level of detail that suits our needs at the time.

At the lowest-detail end of the spectrum, you have ionic bonds - we usually model these as simple Coulombic (a.k.a. charge-charge or electrostatic) interactions. This is good enough to describe bonding in most cases where the bond is between a metal and a non-metal. We model the bond as a complete electron transfer from one atom to another, which results in an electrostatic charge. The Coulomb force between the charges holds the ion together, and so we call it a molecule. What we really mean is "these atoms are now stuck together."

As we move along the periodic table and the metal atoms become less metallic and the non-metals become more metallic, at some rather arbitrary point we say "the ionic approximation is no longer good enough" and start seeing the bond as covalent. What we really mean is that the electrons are not fully transferred between atoms, and so charge-charge interactions don't explain bonding well enough. At that point we can no longer ignore quantum mechanics, and have to acknowledge that the electrons exist in molecular orbitals, not atomic orbitals.

That is the main difference between ionic and covalent bonds. As for dipole-dipole interactions, it again goes back to what the electrons are doing. In a molecule with a dipole moment, what this means is that within the molecule there are polar covalent bonds, and the geometry is such that the electron density on the surface of the molecule is lopsided - one side has more electrons than the other. The dipoles can align and will be attracted to each other via the Coulomb force, but this is not the same as how we model an ionic bond - in that case, one atom gives up one or more electrons completely. In a molecule with a dipole moment, one (or more) atoms just gets "more" of the electron density. Then the two separate molecules, each with their own dipole, are attracted to each other.

We call ionic bonds "intramolecular" forces because they are what hold molecules together - without ionic bonds, you couldn't have a salt crystal, for example. When you break apart sodium chloride, you get sodium ions and chloride ions.

We call dipole interactions "intermolecular" because they are what make separate molecules stick together. When you pull apart two water molecules, you still have two water molecules.

Thinking about it this way, an easy way to distinguish between intermolecular and intramolecular forces is to ask yourself whether you still have the same substance after pulling things apart. If you do, it must have been an intermolecular force. If you don't, it must have been a bond or an intramolecular force.

note - For very large molecules like proteins it gets a little tricky - you can have forces between sites on the same molecule that act like what we normally describe as intermolecular forces.

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  • $\begingroup$ Thank you for the reply. I respect and understand the answer, but I'm not sure. Using your example of intramolecular forces: if you take just one NaCl molecule, it is just that. Just a NaCl molecule. Together, they make a NaCl crystal lattice. However, this is the same with solid H2O; ice. A single H2O molecule is just that. However, if you put more and more together, it, too, forms a crystal lattice. This being said, ionic bonds and intermolecular forces seem to make up the structure of molecules while covalent bonds form the structure of just one molecule (except covalent-network solids) $\endgroup$ – Dylan Spano Jun 9 '15 at 0:02
  • $\begingroup$ Adding onto that, since I ran out of space, it seems ionic bonds can determine the structure of one molecule and the structure of the molecules together. Meanwhile, dipole-dipole does the structure of the whole molecule, and covalent bonds determine the structure of a single molecule, discluding covalent networks. From this perspective, an ionic bond is both an intermolecular force and a chemical bond. $\endgroup$ – Dylan Spano Jun 9 '15 at 0:04
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    $\begingroup$ Ions don't form molecules in the sense you are thinking of, they form what are called "formula units" - the lowest whole number ratio of elements in the ionic solid. I think you understand the physics: dipole interactions and ionic interactions are both electrostatic in nature. Where you are getting hung up is on the nomenclature. A molecule is two or more atoms joined by a covalent bond. By definition, ionic bonds can't be intermolecular forces. $\endgroup$ – thomij Jun 9 '15 at 4:08
  • $\begingroup$ I don't want to hear that something cannot happen "by definition." What I am trying to say is that the definition doesn't seem right. $\endgroup$ – Dylan Spano Jun 9 '15 at 10:47
  • $\begingroup$ Well, good luck with that approach! $\endgroup$ – thomij Jun 9 '15 at 12:02

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