The five most abundant elements in the Earth's crust are oxygen, silicon, aluminum, iron and calcium, in that order. Sodium, potassium and magnesium are also prevalent in the Earth's crust. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur are abundant elements in biological systems, commonly referenced using the memory device CHNOPS. Air is made mostly of oxygen and nitrogen with small amounts of other gaseous elements, such as carbon dioxide.
Oxygen makes up about 46.6 percent of the Earth's crust, and silicon comprises 27.7 percent. Aluminum represents 8.1 percent, while iron makes up 5 percent of the crust of the Earth. Calcium is 3.6 percent of the Earth's crust's chemical makeup.
Without the element carbon, living organisms cannot survive. Carbon lends itself to great structural variability, because it possesses the unique ability to form up to four bonds with several other elements, as well as with itself. Carbon can form double and triple bonds with atoms of elements, further increasing its structural versatility.
Oxygen is the single most abundant element on Earth, while hydrogen is the most abundant element outside of Earth, comprising 92 percent of the overall universe. Stars are giant balls of hydrogen, and the element serves as their fuel source to continue burning. As stars burn hydrogen, they convert it into carbon, oxygen and iron. Large stars produce heavier elements. For instance, a supernova occurs when a star makes an extremely heavy element, such as gold.