Common and rare elements on Earth

Earth is made of many chemical elements. The most common elements are iron, oxygen, silicon, and magnesium. The most rare elements are thallium and bismuth.^[1] Earth also has very small amounts of rarer elements like astatine, but these are radioactive elements that do not last long enough to be part of chemicals or minerals without radioactive decay changing them into more common elements.

Elements are not spread equally out through the Earth. Most of Earth's iron is found in Earth's core, along with many other elements called siderophiles or "iron lovers", which prefer to mix with iron in the core, instead of reacting with oxygen or sulfur to form minerals in the crust or mantle. Elements that react with oxygen easily are called lithophiles or "rock lovers", while elements that prefer to react with sulfur are called chalcophiles or "ore lovers".

To show how elements associate with each other, scientists divide the Earth into parts called compositional layers based on the chemicals in them. The simplest of these divisions separates Earth's core, Earth's mantle, Earth's crust, the ocean or Earth's hydrosphere, and Earth's atmosphere. The mantle and crust are more similar to each other than the core, so they are sometimes combined into the silicate Earth.

The most common elements are not always the easiest elements to use. Elements in the core or mantle are not accessible for mining, while elements in the atmosphere or ocean need to be separated from mixtures or solutions. Most elements are found in chemical compounds that need to be smelted or otherwise refined to make the pure element.

Earth's crust

The most common elements in the crust are oxygen, silicon, and aluminium. Elements that react with oxygen and silicon to make silicate minerals are called lithophiles. These minerals are less dense than the ones found in the mantle, so they float on top to make the crust.