0.9 Energy and polarity of covalent chemical bonds  (Page 7/8)

It is also possible to measure the magnitude of the dipole moment. A number of these are given in [link] for some simple molecules. We can see that, not only does HF have a dipole moment, but in fact it has the largest dipole moment of the molecules listed. What does it mean that HF has a dipole moment? An HF molecule must have a permanent negative end and a permanent positive end. Experimentation shows that the F end of the molecule has a net negative charge and the H end has a net positive charge.

How can this be so? The total number of electrons and protons in the molecule are evenly matched, of course, and the electrons are moving rapidly about the two nuclei just as in H ₂ . Perhaps the electrons are not moving uniformly around the H and the F nuclei. To observe this, we look at the molecular orbital for the shared electrons in the HF bond, shown in [link] .

We observe that the electrons in the molecule move with greater probability near the F atom. There is thus more electron charge around the F nucleus than the positive charge on the F nucleus. And the opposite is true for the H end of the molecule. Apparently, when an H atom and an F atom share electrons, they do not share them equally!

This is an extremely important result, one of the most useful in all of Chemistry. Different atoms have different tendencies to draw electrons to themselves when sharing electrons in a covalent bond. The relative strength with which an atom draws electrons to itself in a bond is called “electronegativity.” A very electronegative atom will attract the shared electrons more strongly than a less electronegative atom. This will produce a negative charge near the more electronegative atom and a positive charge near the atom other. This will, in most cases, create a permanent dipole moment in the molecule. At this point, we don’t know how much negative charge is near the more electronegative atom, but it probably is not the full charge on a single electron since the electrons are still mostly shared by the two atoms. As such, we label the negative charge by δ-, where δ is some number between 0 and 1. In most cases, we don’t even need to know how large δ is. The positive end of the molecule is labeled by δ+, since whatever the negative charge on the negative end must equal the positive charge on the positive end. HF is a good starting example. The F atom is more electronegative than the H atom; hence, the HF molecule has a dipole moment, which makes HF what we call a polar molecule.

Of course, we next want to know why the F atom is more electronegative. We need more data to develop a model for electronegativity. [link] listing the dipole moments of several molecules provides good data for comparison.

Let’s first study the set of molecules HF, HCl, HBr, and HI. It is easy to see that the dipole moments of these molecules are in the order of HF>HCl>HBr>HI. To analyze this comparison, we need to remember that the dipole moment measures the product of the amount of charge separated times the distance by which the charges are separated. Since the dipole moment of HF is larger than that of HCl, let’s compare these two. How do the properties of F atoms compare to those of Cl atoms? One thing we can say for sure: Cl is a larger atom than F. This means that the HCl bond (127 pm) is longer than the HF bond (92 pm). Since the dipole moment depends on the distance between the positive and negative charges, the comparison of bond lengths would perhaps lead us to predict that HCl would have a larger dipole moment than HF, But that’s not what the data tell us. There is only one way to explain this. The difference between HF and HCl must be in the amount of charge that is separated. It must be true that the negative charge on the F atom is greater than the negative charge on the Cl atom. This means that F attracts the shared electrons in the H-F bond more than the Cl atom attracts the shared electrons in the H-Cl bond. This means that F is more electronegative than Cl.