No, both the core and mantle are more dense than the crust. The crust is the least dense layer.
No the mantle is denser than the crust.
The earth's crust is less denser than the mantle.
Aluminum is a light metal. Found almost exclusively in the crust. The heavier metals are found in the core, which is mostly made up of iron.
Due to Earth's gravity, in the past history of Earth denser materials tended to sink down to the center, while less dense materials tended to float to the outside.