If the atomic number of an element increases, then density will increase as well because...?

Not necessarily—(although generally so)—Lead (82) is less dense than Osmium (76) at STP.

Mostly because, “it’s complicated” =)

I mean…generally.

chemistry classes are long gone, but i think it largely depends on the atomic mass and the number of electrons in the outer shell which will repel electrons of adjacent atoms.
but that is just my guess.

Here is a table of the elements sorted by their densities. I’m going to agree with grumpyfish. The reason is because, it is complicated.

@Les: great link!

Part of it is this: http://en.wikipedia.org/wiki/Lanthanide_contraction which affects certain elements density, mainly having to due with (if I’ve understood correctly) that the 4f electron level has some “funny properties”

well..i understand that…its just at a high school level, so mostly generic, adn we’re only talking about the p sublevels.

@smile1 indeed—the important thing to remember (or learn?) is that it’s very hard to make generalizations about chemistry.

E.g., take the “things in a column are generally alike” Nitrogen & Phosphorus are similar to Arsenic chemically, but Nitrogen won’t kill you (directly).

@smile1 The electron density IS a direct function of Z and A , utilizing Avogadros constant as well. It is an important parameter in medical imaging… e.g. CT scanners produce an image which is approximately a image of electron density spatial distributions in the body…. and they look very different in ‘diseased’ or ‘broken’ anatomic sites….......