The periodic table organizes all known elements by their increasing atomic number and recurring chemical properties. While it primarily highlights chemical behavior, it also maps the physical states—solid, liquid, or gas—in which elements exist under standard laboratory conditions. Chemists classify an element’s physical state using Standard Temperature and Pressure (STP), defined as \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) and one atmosphere (atm) of pressure. The vast majority of elements are solids at this reference point; only a small collection exists naturally as gases.
The Eleven Elements That Are Gases
Eleven elements maintain a gaseous state at Standard Temperature and Pressure (STP). These elements fall into two categories based on how they exist as gas particles. The first group consists of six monatomic elements—single, independent atoms. These are the noble gases: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn).
The second group includes five non-metallic elements that exist as diatomic molecules (two atoms bonded together). This group includes Hydrogen (\(H_2\)), Nitrogen (\(N_2\)), Oxygen (\(O_2\)), Fluorine (\(F_2\)), and Chlorine (\(Cl_2\)). All eleven elements have very low boiling points, keeping them in the gaseous phase even at \(0^\circ\text{C}\).
Locating the Gaseous Regions on the Periodic Table
The location of gaseous elements follows a clear pattern defined by chemical families. The largest cluster of gases resides in the far-right column, Group 18. This group, the Noble Gases, is entirely composed of gaseous elements, from Helium to Radon. This column represents one distinct area of gaseous elements.
The remaining five non-metal gases are scattered in the upper-right section of the table, above the metalloid stair-step line. Nitrogen, Oxygen, Fluorine, and Chlorine are grouped in the upper sections of Groups 15, 16, and 17, respectively. Hydrogen is the one element that breaks this upper-right pattern, sitting uniquely in the upper-left corner in Group 1. Despite its placement with the alkali metals, Hydrogen’s chemical properties and low atomic mass ensure it exists as a gas. Overall, the gaseous elements are confined to the extremities of the table, while the bulk of the chart is occupied by solids.
Atomic Structure and the Gaseous State
The gaseous state of these elements relates directly to their atomic structure and the forces between their particles. For a substance to be a gas, the kinetic energy of its atoms or molecules must be greater than the attractive forces pulling them together. The attractive forces between gas atoms or molecules are very weak compared to the strong metallic or network covalent bonds found in solids.
These weak attractions are primarily a type of intermolecular force known as London Dispersion Forces. These forces result from temporary shifts in electron density, creating a momentary, weak attraction to a neighboring particle. Because gaseous elements have very low atomic or molecular masses and small electron clouds, these dispersion forces are minimal. This weak attraction means very little thermal energy is needed to keep the particles far apart and moving freely, the defining characteristic of the gaseous state.