Final answer:
The heaviest elements, iron and nickel, sank to form Earth's core due to the process of planetary differentiation when the Earth was molten.
Step-by-step explanation:
The heaviest elements, including iron (Fe) and nickel (Ni), sink in the early molten Earth to form the core.
This process is known as planetary differentiation, where materials segregate by density under the force of gravity.
During the early stages of Earth's formation, the planet was hot enough to melt, which allowed the heavier elements such as iron and nickel to sink towards the center and form the metallic core.
The lighter silicates remained near the surface, forming the Earth's crust and mantle.