Oxygen (O)

• Oxygen is by far the most abundant element in the earth's crust. Scientists estimate oxygen comprises nearly half of the mass of the crust. It also accounts for 21 percent of Earth's atmosphere. Oxygen is a highly reactive element capable of combining with most other elements. For example, oxygen and iron (Fe) form various compounds we know as iron ore.
  
  

Silicon (Si)

• As the second most abundant element in the Earth's crust, silicon accounts for over 28 percent of its mass. Combined with oxygen, silicon dioxide is the most common compound in the crust. Most people know silicon dioxide as common sand, but it can also take the form of quartz and other crystalline rocks. Silicon is also an essential material in the manufacture of electronics and computer chips.
  
  

Aluminum (Al)

• Aluminum is the third most common element in the Earth's crust. Aluminum is the crust's most abundant metal, but all the earth's aluminum has combined with other elements to form compounds, so it is never found free in nature. Aluminum oxide is a common aluminum compound. Aluminum and aluminum alloys have a variety of uses from kitchen utensils to aircraft manufacturing.
  
  

Iron (Fe)

• Iron is one of the most common and cheapest of all metals and accounts for over 5 percent of the Earth's crust, making it fourth on the list of abundant elements. Iron combined with carbon makes steel. There is archaeological evidence that humans have used iron for thousands of years.
  
  

Calcium (Ca)

• Calcium is the fifth most abundant element in the Earth's crust. Calcium makes up over 4 percent of the crust.. Calcium is another reactive element that is not found free in nature because it readily forms compounds with oxygen and water. Manufacturers use calcium compounds in many applications including gypsum board (drywall), chalk and toothpaste.
  
  

Sodium (Na)

• Sodium may be best known as part of the compound that makes table salt, or sodium chloride, but it also composes over 2 percent of the Earth's crust, making it the sixth most abundant element. Sodium is never found free in nature due to its high reactivity. It is an ingredient in many useful compounds such as baking soda, caustic soda, and borax. Sodium lamps produce a bright yellow-orange light and are widely used to light roads and parking lots.
  
  

Magnesium (Mg)

• Magnesium makes up over 2 percent of the Earth's crust. In nature, magnesium is found in compounds with other elements. It is never found free. Magnesium has many applications in industry and the home. It is the essential ingredient of Epsom salts and is also used as an antacid and laxative. Magnesium-aluminum alloy is used in the construction of aircraft and other applications where strong, light metals are required.
  
  

Potassium (K)

• About 2 percent of the Earth's crust is composed of potassium. This extremely reactive element is never found free in nature. Potassium forms many useful compounds that are used in the manufacture of fertilizer, soaps, detergent and some types of glass.