Carbon, hydrogen, and oxygen are the three main elements that make up many important compounds. Carbon forms chains and rings and bonds easily with other atoms. Hydrogen is the most reactive element and exists as a colorless and odorless gas. Oxygen is necessary for life but also poisonous, existing as a colorless and odorless gas that readily releases energy. Common compounds include water, salt, sugar, and iron. Nutrients include fat, protein, vitamins, minerals, carbohydrates, and trace elements needed in small amounts.