Chlorine gas (\(\text{Cl}_2\)) is not classified as a chemical compound. It is correctly identified as a diatomic element and a molecule. Understanding the difference between elements and compounds is necessary to explain why \(\text{Cl}_2\) does not meet the criteria of a compound. The distinction lies in the type and variety of atoms chemically bonded together.
Elements, Atoms, and Molecules
The atom is the most basic unit of matter, retaining the chemical properties of an element. An element is a pure substance consisting of only one type of atom, and it cannot be broken down into simpler substances by chemical means. Examples include oxygen (\(\text{O}\)), hydrogen (\(\text{H}\)), and chlorine (\(\text{Cl}\)), defined by the number of protons in the nucleus.
A molecule describes two or more atoms that are chemically bonded together. These atoms can be identical, like nitrogen gas (\(\text{N}_2\)), or different, like water (\(\text{H}_2\text{O}\)). The term molecule encompasses both the pure forms of some elements and all chemical compounds. The existence of a chemical bond between atoms is the defining characteristic of a molecule.
The Defining Feature of a Chemical Compound
A chemical compound is a molecule that must consist of two or more different elements chemically bonded together in a fixed, definite ratio. This fixed composition gives compounds their consistent and predictable properties. For instance, water is always \(\text{H}_2\text{O}\), containing two hydrogen atoms for every one oxygen atom.
The properties of a compound are entirely new and distinct from the properties of the elements from which it is formed. Consider table salt (\(\text{NaCl}\)), which is a stable, white, crystalline solid essential for life. It is formed from sodium, a highly reactive metal, and chlorine, a toxic, yellow-green gas. The resulting compound has properties completely unlike its constituent elements, illustrating the transformation that occurs through chemical bonding.
Classifying Chlorine Gas (\(\text{Cl}_2\))
Chlorine gas is composed of two chlorine atoms bonded together. Because this substance is made up of only one kind of atom—chlorine—it satisfies the definition of an element. It is specifically categorized as a diatomic element, meaning its natural, stable form consists of molecules with two identical atoms.
Since it is composed of two atoms chemically bonded, \(\text{Cl}_2\) is also correctly classified as a molecule. However, it does not meet the strict requirement of a compound, which demands the presence of two or more different types of atoms. The bonding of two identical chlorine atoms does not create a new substance with fundamentally different chemical properties, which is the hallmark of forming a compound. Therefore, \(\text{Cl}_2\) is an element and a molecule, but it is not a compound.