Why do less dense compounds make up earths crust while the densest componds make up the core?

Answer 1

Pressure, my dear, pressure. The deeper you go the higher the pressure is. This results in mineral phase changes, such as olivine assuming the spinel structure or quartz converting to shistovite (spelling?), which are more dense minerals. Also, the inner core is solid and the outer core is liquid, because solid is denser than liquid at that pressure/temperature.