There isn't a single chemical formula for Earth. Earth is a complex system made up of many different elements and compounds. Here's a breakdown:

* By mass: The Earth's composition is primarily:

* Iron (Fe): Around 32.1%

* Oxygen (O): Around 30.1%

* Silicon (Si): Around 15.1%

* Magnesium (Mg): Around 13.9%

* Sulfur (S): Around 2.9%

* Nickel (Ni): Around 2.4%

* Calcium (Ca): Around 1.5%

* Aluminum (Al): Around 1.4%

* Other elements: Around 1.2%

* By layers: Earth is divided into several layers, each with its own composition:

* Crust: Primarily composed of oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

* Mantle: Primarily composed of silicate minerals, including olivine and pyroxene.

* Core: Primarily composed of iron and nickel.

Therefore, it's impossible to represent Earth's composition with a single chemical formula. It's a dynamic and complex system with a vast range of elements and compounds.