Um, water weighs 1g/cm3. He means 5 particles.

No he means 1 particle per that volume, or maybe that many particles per cm^3. The numbers you suggest are more dense than water.

alright, so 5 particles per cm^3 to 80 particles per cm^3, not grams (as density is normally measured).

i'm not sure why the downvotes were necessary though when I made it clear I was trying to figure out what he's saying.