An element is a pure substance consisting only of atoms with the same number of protons. Elements can exist in one of three common physical states—solid, liquid, or gas—depending on the ambient temperature and pressure. Out of the 118 known elements, only a small group naturally exists as gases under typical conditions. The state of an element is determined by the balance between the kinetic energy of its particles and the attractive forces holding them together.
Defining Standard Conditions
To determine an element’s physical state consistently, scientists refer to a set of standardized parameters. The concept of “room temperature” in chemistry is often referenced against Room Temperature and Pressure (RTP). RTP is generally defined as a temperature of 25°C and a pressure of one atmosphere. The physical condition of an element is directly related to the strength of the forces between its individual atoms or molecules.
Elements that are gases at this temperature possess extremely weak intermolecular forces (IMFs) of attraction. These forces, primarily London Dispersion Forces, are temporary attractions that occur due to the constant motion of electrons. Because the attractive forces are so slight, the atoms or molecules have enough kinetic energy to remain separated. This allows them to move freely and randomly, exhibiting the characteristic properties of a gas.
The Reactive Diatomic Gases
Five elements are gases at room temperature and exist as reactive diatomic molecules, meaning they bond with a second atom of the same element. These five are Hydrogen (\(\text{H}_2\)), Nitrogen (\(\text{N}_2\)), Oxygen (\(\text{O}_2\)), Fluorine (\(\text{F}_2\)), and Chlorine (\(\text{Cl}_2\)). Their tendency to form a two-atom molecule is due to their high reactivity and the desire to achieve a stable electron configuration.
These diatomic molecules are small and nonpolar, resulting in very weak London Dispersion Forces holding one molecule to the next. Nitrogen (\(\text{N}_2\)), for example, forms a strong triple bond between its two atoms, but the attraction between separate \(\text{N}_2\) molecules is exceptionally weak. This characteristic allows \(\text{N}_2\) to make up about 78% of the Earth’s atmosphere as a gas at ambient temperature. In contrast, the next halogen element, Bromine (\(\text{Br}_2\)), is a liquid because its larger size results in significantly stronger intermolecular forces.
The Noble Monatomic Gases
Six other elements exist as gases at room temperature, differing in structure and reactivity from the diatomic gases. These are the noble gases, which exist as single, monatomic atoms because they are chemically inert. Their outer electron shells are completely full, making them stable and unwilling to bond with other atoms.
The six noble gases are:
- Helium (\(\text{He}\))
- Neon (\(\text{Ne}\))
- Argon (\(\text{Ar}\))
- Krypton (\(\text{Kr}\))
- Xenon (\(\text{Xe}\))
- Radon (\(\text{Rn}\))
This single-atom structure and lack of polarity mean they exhibit the weakest intermolecular forces. The London Dispersion Forces acting between atoms are minimal, requiring temperatures near absolute zero to condense them into a liquid. This inertness and minimal attraction ensure they remain in the gaseous state under standard conditions. Combining these six monatomic elements with the five reactive diatomic gases confirms that a total of eleven elements are naturally gases at room temperature and standard pressure.