1. After several million years, the Earth separated into several layers. Iron, nickel and other heavy metals mostly settled to the core; lighter elements remained in the mantle around the core. The lightest elements, such as oxygen and silicon, floated to the top and cooled, forming a solid crust.
2. The inner core is the most dense. As the Earth formed, the heavy elements migrated towards the center through gravitational compression. This became the Earth’s inner core.
(Hope this helps!)