The question of whether “gas” is an element or a state of matter stems from confusion between a substance’s chemical identity and its physical form. A gas is a physical state, not a chemical element. Substances are classified chemically as elements, compounds, or mixtures, while their physical existence is described by states of matter, such as solid, liquid, or gas. The term “gas” describes the physical behavior and arrangement of a substance’s particles under specific conditions of temperature and pressure. Any substance, whether an element or a compound, can potentially exist in the gaseous state.
What Defines an Element
A chemical element is the fundamental unit of matter that cannot be broken down into a simpler substance through ordinary chemical reactions. An element’s identity is uniquely determined by the number of protons in its atomic nucleus, known as the atomic number. Every atom of a specific element, such as oxygen or gold, has the same number of protons. These substances are systematically organized on the Periodic Table by increasing atomic number.
The concept of an element defines composition and structure at the atomic level. All matter is constructed from these basic building blocks; 118 are currently recognized. Although an atom consists of subatomic particles, the elemental substance cannot be chemically separated into anything simpler. An element represents a specific type of matter defined by its nuclear makeup.
What Defines the Gas State
The gas state is one of the distinct physical forms that matter can take, alongside liquid and solid. This state is characterized by particles, whether atoms or molecules, that are widely separated and move randomly at high speeds. Unlike solids or liquids, a gas has no fixed shape or definite volume. Instead, it completely expands to fill and take the shape of any container it occupies.
The particles in a gas possess significant kinetic energy, overcoming the weak attractive forces between them. This large amount of empty space makes gases highly compressible, unlike liquids and solids. The physical properties of the gaseous state, such as volume and pressure, are sensitive to changes in temperature. As temperature increases, the kinetic energy of the gas particles increases, causing them to move faster and collide more frequently.
Elements that Exist as Gases
Eleven elements on the Periodic Table exist naturally as gases at standard temperature and pressure (STP), defined as 0° Celsius and 1 atmosphere of pressure. These elements are clustered on the right side of the Periodic Table, with the exception of hydrogen. Their existence demonstrates that an element, a specific chemical identity, can exhibit the physical properties of a gas.
The noble gases are all gases. These elements exist as monatomic gases, meaning their particles are single, individual atoms that do not typically bond with others. Their stable electron configuration makes them chemically unreactive, contributing to their gaseous nature. The noble gases include:
- Helium (\(\text{He}\))
- Neon (\(\text{Ne}\))
- Argon (\(\text{Ar}\))
- Krypton (\(\text{Kr}\))
- Xenon (\(\text{Xe}\))
- Radon (\(\text{Rn}\))
In contrast, the other gaseous elements exist as diatomic molecules, meaning two atoms of the same element are chemically bonded together. Whether monatomic or diatomic, these substances are still chemically defined as elements because their molecules contain only one type of atom, yet they physically behave as gases. These diatomic elements are:
- Hydrogen (\(\text{H}_2\))
- Nitrogen (\(\text{N}_2\))
- Oxygen (\(\text{O}_2\))
- Fluorine (\(\text{F}_2\))
- Chlorine (\(\text{Cl}_2\))
Compounds that Exist as Gases
Illustrating that “gas” is a state of matter, many chemical compounds also exist in the gaseous state under normal conditions. A compound is a substance formed when two or more different elements are chemically bonded together. Gaseous compounds are characterized by weak intermolecular forces and low molecular masses.
Common examples include carbon dioxide (\(\text{CO}_2\)), formed from one carbon and two oxygen atoms. Methane (\(\text{CH}_4\)), the primary component of natural gas, is a compound of carbon and hydrogen that exists as a gas. Even water, a compound of hydrogen and oxygen, exists as water vapor or steam when its temperature is raised sufficiently. The fact that substances with varying chemical compositions can all exist as a gas confirms that the term describes a universal physical condition of matter.