Confused about London dispersion forces....! Help?

Question

Is the London dispersion force greater between molecules of hydrogen sulfide (H2S) or molecules of water (H2O)? I know that the London dispersion force is supposed to increase as atomic mass and the number of electrons increase, and hydrogen sulfide has more electrons in all than does a molecule of water. I also know that the London dispersion forces have to do with creating temporary dipoles in non polar molecules, but should I take the polarity of polar molecules into consideration when trying to determine the relative strengths of the London dispersion forces? So does the fact that oxygen has a greater electronegativity than sulfur affect the strength of the London dispersion forces? I am confused. All my book talks about is changing concentrations of electrons in non polar molecules.

Answer 1

Well, the EN trends on the periodic table tell us that within a column, the smaller the atom the greater the electronegativity. But the "rule" is that dispersion forces (i.e., polarizeability of the atom) increase with increasing atomic radius. It seems to me that even if EN value does affect the magnitude of the dispersion, that this would already be taken into account by the rule about dispersion forces.



You don't take the polarity of the molecule itself into consideration when talking about dispersion forces because the intermolecular forces that are present due to polarity are called something else: dipole-dipole forces. It's a somewhat artificial distinction, to be sure. But so it is with "rules" and definitions - it's the nature of the categorizing natural phenomena.

Answer 2

There are 2 kinds of intramolecluar forces (those that glue atoms into molecules): ionic and covalent



There are three kinds of intermoleculer forces (those that attract molecules to molecules):



-dipole-dipole - these occur only in polar molecules. The larger the electronegativity differences the more polar these molecules can be and the more sticking power they have with other polar molecules. In order to determine if a molecule is polar you must look at both the electronegativity of the atoms and the geometry of the molecules, especially you must also notice lone pairs of electrons.

-Hydrogen bonding - this is really a form of dipole dipole it gets a special name because it involves the strongest of these due to the high electronegativiey difference between H and O,N,F but it is a dipole dipole force.

-LDF or london dispersion forces also known as Van Der Waal forces. These occur in all molecules BUT they are fairly weak and are noticed the most in nonpolar molecules (only because of the complete absense of dipole-dipole forces.) Picture a non polar molecule like I2. Iodine preffers to exist as a diatomic molecule and the electron count between the two is108, that is a lot of electrons. Now picture where those electrons might possibly at any one moment. It is true that they will want to be as far apart as possible but sometimes, it is just a fluke they might mostly end up on one side of the molecule, that is the LDF force. Now picture a positively charged something floats near to the I2 molecule, maybe more of the 108 electrons will move towards the positively charged thing. Now what does it look like? A teardrop. The reason larger non polar molecule have greater LDF forces is because they have more electrons to move around. For instance Helium only has 2 electrons that can be on one side or the other compared to diatomic ioidine.



Just remember that pure water has two intermoleculer forces hydrogen bonding (really a form of dipole-dipole) that is a strong force and a small LDF because it has 8 electrons (and you are right mostly those electrons will hang near the oxygen!)



In the big scheme of things intramolecular forces are on the order of 100 times stronger than intermolecular forces.

Answer 3

It probably is. However, the London dispersion force is not a very important force factor in polar molecules such as H2O and to a lesser extent in H2S.

Answer 4

First of all clear your concept; London Dispersion force only be exhibited by non polar molecules in which there is concept of induced dipole/polarity , in polar molecules other van der waal's forces come into play.