why do more electrons increase dispersion forces?

More electrons usually means a larger size and that means higher dispersion forces in general.

why? or maybe the right question is how does the size contribut to the forces?

The larger an atom the farther the electrons are away from the nucleus; therefore, the easier it is to distort them. Here is a site that talks about that.

http://www.chem.purdue.edu/gchelp/liquids/disperse.html

Dispersion forces, also known as London dispersion forces, are the weakest intermolecular forces that exist between molecules. They originate from temporary fluctuations in electron distribution, leading to the formation of instantaneous dipoles. These induced dipoles can then induce dipoles in neighboring molecules, resulting in an attractive force between them.

Now, coming to your question, when we consider molecules with more electrons, it means the electron clouds of these molecules are larger and more easily polarizable. This increased electron cloud size enhances the probability of temporary fluctuations, resulting in larger instantaneous dipoles. Consequently, the induced dipoles in neighboring molecules will also be larger.

With larger and more pronounced dipoles, the magnitude of dispersion forces increases. Thus, more electrons generally lead to stronger dispersion forces between molecules.

To understand this concept further, you may refer to the following steps:

1. Identify the molecules involved: Determine the nature of the molecules.

2. Analyze electron distribution: Consider the number of electrons in each molecule and assess the size of their electron cloud.

3. Assess polarizability: Identify the ease with which the electron cloud of each molecule can be distorted temporarily.

4. Determine induced dipoles: Evaluate the magnitude of the induced dipoles in neighboring molecules.

5. Compare dispersion forces: Compare the strength of dispersion forces based on the size of induced dipoles.

By considering these steps, you can gain a better understanding of why more electrons increase dispersion forces.