Chemical differentiation describes the process by which heavier, molten elements like iron and nickel sank to the center of the Earth and lighter elements like silicon, oxygen and aluminum populated the surface of the planet 4.5 billion years ago, according to Indiana University-Purdue University Fort Wayne. Scientists theorize that gravity caused the chemical elements to separate into different parts of the Earth.
The inner core and outer core of the planet consist of iron and nickel. The inner core is solid and the outer core is liquid, and these two spaces take up more than half of the Earth's radius. The mantle sits outside of the core and is made of layers of calcium, magnesium, iron, silicon, oxygen and aluminum. The mantle contains the largest portion of the Earth's volume.
Scientists have never taken direct samples from the mantle or core. However, vibrations from earthquakes were measured after they traveled through the core. These measured vibrations are compared to laboratory tests and physical calculations.
A study posted to Cornell University's website reveals that the Earth is 18 percent hydrogen by weight. This paper also theorizes about the elements abundant within the inner planets. The chemical differentiation of the outer gas giant planets varies from that of the inner planets of the solar system.