0.9 Energy and polarity of covalent chemical bonds  (Page 6/8)

Observation 3: dipole moments in diatomic molecules

We now have an understanding of what it means for two atoms to share an electron pair and why this results in a bond. In addition to H ₂ , this description works well in describing and understanding chemical bonds such as in F ₂ or Cl ₂ . More work is required to understand multiple bonds such as the double bond in O ₂ , but it turns out that the same principles apply. What about molecules where the atoms are not the same, e.g. HF? The Lewis model of these molecules still assumes a sharing of an electron pair. But we learned in our study of atomic structure that the properties of H atoms and F atoms are quite different. For example, the ionization energy of an F atom is larger than the ionization energy of an H atom. F atoms also have very strong electron affinity. Do such different types of atoms share electrons? If so, does it matter that the properties of the two atoms are so different?

We need observations to answer these questions. First, we can do as we did with H ₂ and examine the energies of the bonds between different atoms. The bond energy of HF is 568 kJ/mol, larger than the bond energy of H ₂ . By contrast, the bond energy of F ₂ is 154 kJ/mol, quite a bit weaker than the bonds in either HF or H ₂ . The bond energy of HCl is 432 kJ/mol, weaker than the HF bond. The energy of one O-H bond in H ₂ O is 463 kJ/mol. Clearly, the strength of bond depends on what types of atoms are bonded together. This suggests that the sharing of an electron pair depends on the properties of the atoms in the bond, including the sizes of the atoms and the charges on the nuclei. In fact, it would not be too surprising if different atoms did not share the electrons equally.

To find out if this is the case, we observe a property of molecules called the “dipole moment.” An electric dipole is simply a separation of a positive and a negative charge. The dipole moment, usually labeled μ, measures how strong the dipole is by taking the product of the amount of the charge times the separation of the charge. Just having a positive charge, let’s say a proton, and a negative charge, let’s say an electron, does not mean that there is a dipole moment. The hydrogen atom does not have a dipole moment because the electron is in constant rapid motion around the positive nucleus. As such, viewed from outside the atom, there is no positive “end” or negative “end” to the hydrogen atom, so there is no dipole moment. This is true of all atoms.

It might seem that molecules could not have dipole moments for the same reason. The electrons are moving about the nuclei rapidly. In the H ₂ molecule, although there are positive and negative charges, there is no end of the molecule which looks more positive and no end which looks more negative. H ₂ does not have a dipole moment.

What about a molecule like HF? If an H atom has no dipole moment and an F atom has no dipole moment, does HF have a dipole moment? Experimentally, molecular dipoles can be observed in a number of ways. If you put a molecule with a dipole moment in an electric field, it will line up with the field, with the positive end of the dipole pointed towards the negative end of the field. Dipoles can interact with each other as well, so that the negative end of one dipole will point towards the positive end of another dipole.