Question

Does density generally tend to decrease or increase across the table and why?

Answer 1

Obviously, atomic mass increases across the Periodic Table; the same does not apply for density.

Explanation:

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, `"Mass"/"Unit Volume"`.

A better question would be to ask whether density increases down a Group. Start with the halogens. And I don't know the answer.