Here's why:

* Shape and Packing: Pentane has a linear or slightly branched structure, which means its molecules can pack together more tightly. This leads to a higher density. 2,2-dimethylpropane, on the other hand, has a very compact, spherical shape due to the branching at the carbon chain. This spherical shape prevents efficient packing, resulting in a lower density.

* Intermolecular Forces: While both molecules have only London dispersion forces (weak intermolecular forces), the more compact structure of 2,2-dimethylpropane allows for stronger London dispersion forces due to a larger surface area of interaction. This slightly higher intermolecular force contributes to a higher density.

In summary:

* 2,2-dimethylpropane (neopentane) has a higher density than pentane due to its compact, spherical structure, which allows for more efficient packing and stronger intermolecular forces.

Let me know if you'd like more information about density or intermolecular forces!