I don't understand how HCl is not a ionic bond.

[LogicalAcid]

Hydrogen has one valence electron, Chlorine has 7. Chlorine only needs one to become stable, and hydrogen wants to get rid of its only one. When they come in contact, shouldn't they react and form HCl, but held together by ionic bonds?

 

[weesiang_loke]

hi,
For simple explanation, first of all ionic bond only formed between metal and non-metal. But both Cl and H are non-metal . So actually HCl forms polar covalent bond. Cl is large and has very high electronegativity compared to H. Thus the electron cloud tends to move towards Cl. Thats why it is polar.

 

[alxm]

No bond is ionic; ionic bonds don't exist in reality.

 

[johng23]

This sounds to me like a stupid convention where people just don't call it ionic. We call a bond ionic when the electron density is concentrated more near one of the two atoms participating in the bond. If chemists arbitrarily say that it's not ionic because hydrogen is not a metal, that's semantics, not science. Besides, wikipedia says that ammonium has ionic bonds, and those aren't metals.

Correct me if I'm wrong here!

 

[JustinLevy]

We call a bond ionic when the electron density is concentrated more near one of the two atoms participating in the bond.

By that reasoning, every single bond in propylene is ionic.

There clearly is a continuous spectra to these things, so yes a seemingly arbitrary cutoff / convention is usually chosen. Because the extremes are easy to imagine, it gives us insight to the character of the bond to understand how "close" it is to one extreme of the other. So I think the concept still has some use despite the tendencies of some books to turn it into a taxonomy (rarely does everything fit perfectly into labelled groups).

 

[johng23]

By that reasoning, every single bond in propylene is ionic.

So what's wrong with saying that every bond in propylene is ionic? Is the idea that, if it were ionic, the bonds should not have the same directionality?

If that is the deciding factor, then I still agree with the OP that HCl should be considered ionic. It is only two atoms so directionality can't be a factor.

 

[Borek]

You were already told that there is no such thing as purely ionic and/or purely covalent bond. Each bond has a little bit of both. In some cases ionic characteristics dominates (like in NaCl) in some cases covalent characteristics (like in methane). Bonds in propeylene are MOSTLY covalent.

 

[mathman]

One way to tell them apart (although not in all cases) is to see what happens when put into water. HCl will immediately split into H+ and Cl- ions. Hydrocarbons don't ionize.

 

[Redbelly98]

You were already told that there is no such thing as purely ionic and/or purely covalent bond. Each bond has a little bit of both.

Wouldn't the bonds in H₂ or Cl₂ be purely covalent? Or are we just considering bonds between different types of atoms?

 

[Borek]

One way to tell them apart (although not in all cases) is to see what happens when put into water. HCl will immediately split into H+ and Cl- ions. Hydrocarbons don't ionize.

For HCl that's a dangerous approach. Bond in HCl is polarized, but it is covalent enough so that HCl is a gas at STP, while all ionic substances are solid.

Wouldn't the bonds in H₂ or Cl₂ be purely covalent? Or are we just considering bonds between different types of atoms?

I have ignored these molecules for a purpose, LogicalAcid (OP) is too confused even without exclusions and specific cases. But yes, you are right, as the electronegativity difference is zero, these are classified as purely covalent.

 

[johng23]

You were already told that there is no such thing as purely ionic and/or purely covalent bond. Each bond has a little bit of both. In some cases ionic characteristics dominates (like in NaCl) in some cases covalent characteristics (like in methane). Bonds in propeylene are MOSTLY covalent.

I don't know why you're acting like I'm not listening or being dense. Up to this point NO ONE had answered the question. I understand everything you said in this section, and if I didn't know that propeylene is covalent, that's because no one had provided a better way to think of it than the "electron density is closer to one atom" concept, not because I don't understand that an ionic bond is an abstraction.

For HCl that's a dangerous approach. Bond in HCl is polarized, but it is covalent enough so that HCl is a gas at STP, while all ionic substances are solid.

So is this what the confusion boils down to? An ionic bond can only exist in a solid by definition? If that's the case, then it should have been the answer to the original post...
It seems to make some amount of sense anyway.

 

[alxm]

Covalent vs Ionic is an old, pre-quantum theory of chemistry. It's basically just wrong, and would've been dead and buried along with contemporary ideas such as the http://en.wikipedia.org/wiki/Cubical_atom" , if it hadn't been for Linus Pauling (a big fan of cubical atoms, btw), who put a lot of effort into finding theoretical justifications for them from the then-new quantum theory. In retrospect, that effort was mostly political, getting chemists on-board with the new way of thinking. Nobody actually calculates the percentage of ionic-vs-covalent 'character' of a bond anymore, much less uses that number for anything.

Anyway, so if you're talking about covalent-vs-ionic today, you're talking about Pauling's theory. Now, in Pauling's theory, the ionic character can be calculated from the difference in (Pauling) electronegativities between the atoms. So to begin with, there's a sliding scale, and really no such thing as a purely ionic bond, nor any such thing as a purely covalent bond, except for a homonuclear diatomic, where the electronegativities are equal.

However: Pauling electronegativities are defined by the relative difference in homonuclear-diatomic binding energies of the two atoms! The end result is that the characterization of a bond between atoms A and B, in Pauling's theory, amounts to little more than a quantification of how differently elements A and B bind to each other compared to how they bind to themselves. That's why the theory isn't very useful; it doesn't amount to much more than a single-number quantification of how different the two elements are.

More http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291097-461X%281999%2974:2%3C87::AID-QUA5%3E3.0.CO;2-E/abstract" , you can define a 'covalent state' and an 'ionic state' in Valence-Bond theory (Pauling again), and view the thing as a quantum-mechanical superposition of these, but not only is there no binary distinction then, but a bond can't be characterized in terms of two states alone either. E.g. many textbooks claim that VB theory fails at describing the triplet ground-state of O₂, probably because they tried to describe it in two wave functions, properly you need to take into account a three-electron-bond state. (The origin of this myth seems to be due to repetition of an offhand remark by Lennard-Jones.)

johng23 says:

We call a bond ionic when the electron density is concentrated more near one of the two atoms participating in the bond.

That's an interpretation of "ionic character", and unfortunately one that's still commonly taught. I say 'unfortunately' because it's also wrong. You're basically explaining a vaguely-defined property in terms of another vaguely-defined property, because there simply isn't any clear definition or measure of what it would mean for a bonding electron to 'belong' to one atom more than another. There are various quantum-chemical methods of 'population analysis' to arrive at some kind of number for this (e.g. Mulliken populations, Löwdin populations), but one thing they all have in common is that they're pretty useless at describing chemistry. You can't even get a reliable measure of oxidation states from them. (Spin density, on the other hand, does say a lot)

So in summary: Electronegativity represents something real, albeit vaguely defined. The polarity of a bond is quite real, it's easily defined theoretically in terms of electrostatics and can be measured directly as the dipole moment. But things like orbitals, covalent-vs-ionic bonds and resonance structures are entirely theoretical constructs. And out of those, the concept of covalent/ionic is the one that's both least theoretically justified and least useful in describing reality. I'd suggest people just forget about covalent versus ionic, metals versus non-metals, etc, and just talk and think in terms of polarity (which is what most chemists do anyway). HCl has a dipole moment of about 1 D, NaCl has about 9 D, KCl about 10 D. But there's no real reason to consider them to have different bonds; in MO theory they're the same bonds, you just have different effective nuclear charges, and hence different polarity.

(I swear, if these covalent-vs-ionic questions keep popping up, I'm going to end up writing a whole article for J Chem Ed )

 

[johng23]

Thank you! Very helpful.

Even in classes at prestigious schools taught by "successful" professors, you can be given tidbits of pure BS without any insight into the background or the applicability to modern thought.

 

[Borek]

It doesn't want to be as simple. I mean - what alxm wrote is right from the theoretical point of view, but try to explain it to HS students. They have to start with some approximation, ionic vs covalent is not a bad one.

In a way it is similar to difference between chemical vs physical change - some argue distinction is stupid, and they have very serious arguments, but try to explain whole spectrum of differences and nuances in types of bonds and their energies to HS Joe Nottoobright who just started to learn chemistry.