The simplest alkene is ethene, commonly referred to as ethylene.

Ethene, or ethylene, is classified as the simplest alkene due to its minimal composition of carbon and hydrogen atoms. Alkenes are a type of hydrocarbon, meaning they consist solely of hydrogen and carbon atoms. The defining feature of alkenes is the presence of a carbon-carbon double bond.

Ethene has the molecular formula $C_2H_4$, indicating that it contains two carbon atoms and four hydrogen atoms. These two carbon atoms are linked by a double bond, and each carbon is bonded to two hydrogen atoms. This arrangement makes ethene the simplest alkene, as it has the least number of atoms necessary to establish a carbon-carbon double bond.

In terms of its molecular structure, ethene is planar. The carbon-carbon double bond results in a flat configuration, with hydrogen atoms positioned around the carbon atoms in a tetrahedral geometry. This geometry arises from the carbon atoms utilizing $sp^2$ hybrid orbitals, which facilitate the formation of the double bond.

At room temperature and pressure, ethene is a colorless gas. It plays a vital role in various industrial processes, including the production of polyethylene, a widely used plastic. Additionally, ethene functions as a plant hormone that regulates growth and ripening.

Understanding the structure and characteristics of ethene is crucial for grasping the properties of alkenes in general. As the simplest alkene, ethene serves as a fundamental model for the structure and behavior of more complex alkenes.