The physical state of an element is determined by the balance between the attractive forces holding its atoms or molecules together and the kinetic energy of those particles. At standard room temperature, most elements are solids, but a small, distinct group exists naturally in the gaseous state. When measured under these standard conditions, there are precisely 11 elements on the periodic table that exist as a gas.
Identifying the Gaseous Elements
This group of 11 elements can be separated into two distinct chemical families based on their structure. The first group consists of six noble gases: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). These elements are inherently monatomic, meaning they exist as single, independent atoms rather than forming bonds with other atoms. Their full outer electron shells make them chemically stable and unreactive, which contributes to their gaseous state.
The second group is composed of five diatomic molecules, which are elements that exist as pairs of bonded atoms. These include Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), and Chlorine (Cl2). All of these elements are located on the right side of the periodic table, with the exception of hydrogen.
Why Certain Elements Remain Gases
The persistence of these 11 elements in a gaseous state at common temperatures is due to the weakness of the forces between their atoms or molecules. These elements are either nonpolar, like the diatomic molecules, or monatomic, which means they do not possess permanent electrical charges to attract one another. Therefore, the only attractive force present is the very weak London Dispersion Force (LDF), a temporary attraction caused by random fluctuations in the electron cloud.
The kinetic energy of the particles at room temperature is high enough to easily overcome these extremely weak LDFs. For the lightest elements, such as hydrogen and helium, the small number of electrons translates to a very low polarizability, which keeps the LDF nearly negligible. The energy required to condense these substances into a liquid is minimal, resulting in boiling points that are far below zero degrees Celsius.
The Influence of Standard Conditions
The specific count of 11 gaseous elements is entirely conditional upon the precise definition of “room temperature” and pressure. In science, a common reference point is Standard Ambient Temperature and Pressure (SATP), which is defined as a temperature of 25°C and an absolute pressure of 1 atmosphere. This temperature range, typically cited as 20°C to 25°C, is generally what is meant by “room temperature”.
If the temperature is changed even slightly, the number of gaseous elements can be altered. For example, the element Bromine (Br2) has a boiling point of about 58.8°C, meaning a modest increase in temperature above the standard range would cause it to transition from a liquid to a gas. Similarly, the standard reference point called Standard Temperature and Pressure (STP) uses a much colder temperature of 0°C, which would cause some of the 11 gases to liquefy, thus changing the count.