Does density generally tend to decrease or increase across the table and why?
Why not? Because Group 18 of the Periodic Table is the Noble Gases. These are discrete particles that consist of the 1 atom, and are gases not condensed species. The problem arises because molecular structure (if there is one) obviously influences density,
A better question would be to ask whether density increases down a Group. Start with the halogens. And I don't know the answer.