Here's a breakdown of why:

* Crust: This is the outermost layer, composed mostly of lighter elements like silicon, oxygen, and aluminum. Its density ranges from 2.7 to 3.0 g/cm³.

* Mantle: The layer beneath the crust, mostly made of denser minerals like iron and magnesium. Its density ranges from 3.3 to 5.5 g/cm³.

* Outer Core: A liquid layer composed primarily of iron and nickel. Its density is around 9.9 g/cm³.

* Inner Core: A solid ball of iron and nickel at the Earth's center. It has the highest density, around 12.6 g/cm³.

Therefore, the crust, being the outermost layer and composed of lighter elements, has the lowest density compared to the other layers.