* Early Earth was a molten ball: When Earth formed, it was incredibly hot and molten. This heat came from the energy of collisions during its formation and from radioactive decay.

* Density and Gravity: The heavier elements, like iron and nickel, sank towards the center due to gravity. This process is called differentiation. Lighter elements, like silicon and oxygen, rose towards the surface.

* Not just lightest, but heavier: It's important to remember that the lightest elements, like hydrogen and helium, were mostly lost to space during Earth's formation due to the heat.

* Layered Structure: This differentiation process created the layered structure of Earth:

* Core: Mostly iron and nickel

* Mantle: Mostly silicate rocks

* Crust: Lightest, solid rocks

So, it's not that the lightest elements moved to the center, but the heavier elements moved down, leaving lighter elements closer to the surface.