If you took our whole planet and ground it up into a powder, then analysed that powder for its elemental composition, what would you find? A third of the powder would be iron, another third would be oxygen. Of the remaining ~35%, 30% of it is magnesium and silicon. Most people would guess that maybe elements like carbon or hydrogen would be next on the list ... but they'd be wrong. The element that ends up at #5 on the list is sulfur.
Now, this exercise of thinking of a "bulk Earth" shows how biased we are by the surface of our planet when we ponder its composition. Most of the mass of our planet is in the planet's metallic core (iron, mostly). Silicon and magnesium are found in the rocks of the Earth's mantle, the primary rocky component of Earth. Oxygen -- well, it's everywhere, bonding with elements to form a large portion of the minerals from the core to the crust.
