What is the empirical formula of N2O4

Dinitrogen tetraoxide (N2O4) is a reactive chemical compound. It is an oxidizing agent which is commonly used in chemical synthesis or as rocket fuel.

quartz

explanation:

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.