VSEPR Supporting Evidence

The test of a good model, is how well it stands up to experimental data. How well does VSEPR work? We can evaluate several ways. One way we will use is related the polarity of compounds. That is, how well does VSEPR predict the polarity of molecules and how do experimental observations compare. What do I mean by polarity of molecules? A molecule is polar if as a result of unequal sharing of electrons, centers of positive and negative charge do not coincide. Recall in CHEM 1215 we discussed the polarity of chemical bonds in simple binary compounds. The criteria for determining whether a chemical bond is polar or nonpolar was the difference in the electronegativity of atoms involved in the chemical bond. If the two atoms differed in electronegativity the chemical bond was polar. If the electronegativity was the same the bond is nonpolar.

We conclude that F₂ has a nonpolar bond

because the electronegativity is the same for the two atoms sharing the pair of electrons in the bond. For HF the two atoms are different,

Fluorine is more electronegative compared to hydrogen. The result is the electrons in the covalent bond spend more time on the fluorine atom than on the hydrogen atom. This produces a partial negative charge on the fluorine and a partial positive charge on the hydrogen.

This separation of charge produces a small dipole making the molecular polar. The molecule can be thought of as a small magnet with a positive end and a negative end. Opposites attract so HF molecules are attracted to each other when in the liquid and solid phase (but more about that later). The dipole that is produced is represented as a vector with an arrow pointing towards the negatively charged end of the molecule and a positive symbol on the other end of the vector. (The magnitude of an electric dipole is reported as the electric dipole moment, in a unit called the debye (D).)

So we conclude that HF is a polar molecule because the only bond in HF is polar. F₂ is a nonpolar molecule because it does not contain a polar bond. All heteronuclear diatomic molecules are polar and all homonuclear diatomic molecules are nonpolar.

The presence or absence of an electric dipole can be determined experimentally if a sample of the compound of interest is placed in an electric field. If the molecule is polar alignment with an electric field will occur. The net result is the molecules effect the capacitance (the capacity of the plates to hold a charge) of the plates which maintain the electric field. By measuring the capacitance of the plates separated by different chemical substances allows the determination of dipole moments. If a molecule can be characterized as having polar bonds that do not cancel each other out the compound will have an experimentally measureable dipole moment.

What happens in more complicated molecules? It turns out that, depending on the geometry, these individual polar bonds in molecule may cancel each other out, which results in a nonpolar compounds. If the polar bonds in a molecule do not cancel each other the molecule is polar and contains a permanent distribution of electron density which gives rise to a dipole moment.

Lets consider several examples to demonstrate how the geometry of a molecule effects the dipole moment of a compound. We'll begin with CO₂ and H₂O. Both molecule have a central atom, carbon or oxygen, surrounded by two terminal atoms. According to VSEPR CO₂ is linear and H₂O is bent. If we consider CO₂ each C=O bond is polar, because of the difference in electronegativity of the atoms in the bond. However, because the geometry of the two polar bonds are exactly opposite one another and equal in magnitude the net effect is to cancel each other out so that the molecule has no dipole moment, CO₂ is a nonpolar compound.

H₂O, on the other hand, which also contains two polar bonds (O-H) has a molecular geometry that does not allow the contributions of the polar bonds to cancel out. Water has a permanent dipole moment and is a polar compound.

Experimentally CO₂ has no measureable dipole moment, but H₂O (1.87 x10^–18 D) has a very large dipole moment. We will discuss how the polarity of water effects its physical properties in Chapter 14.

We can generalize these two cases.

• Molecules containing central atoms with no lone pair electrons and bonding pairs of electrons to identical terminal atoms are nonpolar,

• Molecules containing central atoms with one or more lone pair electrons are polar. (Note: there are exceptions to this rule for central atoms with electron-pair geometries of trigonal bipyramidal and octahedral. But we will not discuss these exceptions.)

Lets consider two additional examples to demonstrate how the geometry of a molecule effects the dipole moment of a compound. We will consider CCl₄

and CH₃Cl.

Both molecule have a central carbon atom, surrounded by four terminal atoms. According to VSEPR both CCl₄ and CH₃Cl are tetrahedral. If we consider CCl₄ each C-Cl bond is polar, because of the difference in electronegativity of the atoms in the bond. However, because the geometry of the four polar bonds and that all four bonds are equally polar the net effect is to cancel each other out so that the molecule has no dipole moment, CCl₄ is a nonpolar compound. CH₃Cl, on the other hand, which also contains polar bonds (C-H and C-Cl) is also tetrahedral, however because the terminal atoms are non identical the compound has a permanent dipole moment and is a polar compound. This introduces one more rule for predicting polar compounds,

• Anytime the molecule has a central atom with non identical terminal atoms the molecule will be polar.

After all this if I give you the formula of a compound you should be able to

• draw the Lewis structure;

  • then tell me

    • the number of lone pair and bonding pair of electrons on the 'central' atom;

    • the electron-pair and the molecular geometry;

    • the bond angles around any 'central' atom; and

    • if the molecule is polar or nonpolar.

So lets try figuring out the polarity for BF₃ and NH₃. Go here to see what I came up with!