Here are the layers of the Earth in order from least dense to most dense:

1. Atmosphere: The outermost layer, composed primarily of nitrogen and oxygen, is the least dense.

2. Crust: The Earth's solid outer layer, divided into oceanic crust (thinner and denser) and continental crust (thicker and less dense).

3. Mantle: The thickest layer, mostly solid but behaving like a very viscous fluid over long periods.

4. Outer Core: A liquid layer composed mostly of iron and nickel.

5. Inner Core: The Earth's solid center, primarily made of iron and nickel, and extremely dense.

Note: While the inner core is the densest, it is actually the hottest. The extreme pressure from all the layers above it forces the iron and nickel to stay in a solid state despite the intense heat.