Video: Applying Knowledge of the Relationship between the Molecular Geometry, Bond Dipoles, and Net Dipole Moment of CH₄

For statements I and II, state for each if they are true or false. I) The molecule of CH₄ has a net dipole moment of zero. II) The arrangement of atoms in the CH₄ molecule is tetrahedral and symmetrical; hence, the bond dipoles within the molecule cancel each other out. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The molecule of CH4 has a net dipole moment of zero. II) The arrangement of atoms in the CH4 molecule is tetrahedral and symmetrical; hence, the bond dipoles within the molecule cancel each other out. If both are true, state if II is a correct explanation for I.

Dipole moments are due to the separation of charges. In chemistry, this often occurs because of the uneven sharing of electrons within a bond. This occurs due to differences in electronegativity between the atoms in a molecule. For example, in this cartoon I’ve drawn, the electrons are closer to the orange atom on the left than they are to the atom on the right. This means that the orange atom on the left is more electronegative, meaning that it pulls more on the electrons within the bond.

Because the electrons are closer to the atom on the left than they are to the atom on the right. This causes the atom on the left to be partially negatively charged and the atom on the right to become partially positively charged. Due to the separation of charges across this bond, there is a dipole moment, which we indicate with this arrow with a plus sign, where the arrow points towards the more electronegative atom.

If a molecule has a net dipole moment, we call that molecule polar. But if it has no net dipole moment, we call it nonpolar. Unfortunately, it’s not enough to just determine whether or not a bond is polar to figure out if an entire molecule has a net dipole moment. For example, let’s take a look at CO2. The oxygens are both more electronegative than the carbon is. So there would be a partial negative charge on each of the oxygens and a partial positive charge on the carbon. This means that there would be a dipole moment pointing away from the carbon towards each of the oxygens.

But because the dipoles between the carbon and the oxygen are the same strength because the oxygens are both pulling on the carbon the same amount and the molecule is symmetrical, these dipoles cancel each other out. This is similar to how negative one and one cancel each other out on a number line. Even though there’s a dipole moment across each of these bonds in CO2, they cancel each other out. So the overall molecule is nonpolar.

In statement I, we have to figure out if the molecule CH4, which is called methane, has a net dipole moment of zero. Let’s take a look at the molecule of methane. Hydrogen is less electronegative than carbon is. So carbon would pull more on the electrons in the bond than hydrogen would. This would cause there to be a dipole moment, pointing towards the carbon across each of the bonds in methane. Like our example with CO2, all of our bond dipoles are identical. But unlike CO2, this molecule is not in a straight line, which we can see if we look at the more three-dimensional lines and wedges drawing for methane.

But we could imagine rotating this molecule. And we could see that the overall molecule still looks the same after the rotation. So this molecule must be symmetrical. So just like we saw with the example of carbon dioxide, all of these bond dipoles would cancel out. This would leave us with a net dipole moment of zero. So statement I is true.

Statement II says that the arrangement of atoms in the CH4 molecule is tetrahedral and symmetrical. Hence, the bond dipoles within the molecule cancel each other out. Tetrahedral molecules have four bonds around the central atom and no lone pairs. So the methane molecule does have tetrahedral geometry. And as we’ve discussed, because the molecule is symmetrical, the bond dipoles between each of the hydrogens and the carbon would cancel each other out. So statement II is true as well.

Because both statement I and II are true, we have to state if II is a correct explanation for I. So that is, we need to determine if the bond dipoles within methane canceling because the molecule is tetrahedral and symmetrical is the reason that the molecule has a net dipole moment of zero. This is true. As we’ve discussed, because methane is symmetrical, the bond dipoles all cancel each other out. So overall methane has a net dipole moment of zero.

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