Why do greater numbers of people settle in the western part of Russia as opposed to the eastern part? How do the fertile plains, or chernozem, play a role in this as well?

Because the climate of Western part of the country is much more favorable in comparison to the Eastern part. Most of the Eastern part, called Siberia is practically unsettled, because the climate is unfavorable. The average temperature is below zero, and it is practically impossible to crop anything. On the other part, the more you go to west, the climate is more favorable, and also a fertile land where you can crop different vegetables is available.

Explanation:

The chernozem is characteristic not only for Western Russia, but also in the Continental Europe. For example large part of Pannonia is covered with chernozem. That is why this is the part of Russia where agriculture is boosted.