Group 18 on the periodic table represents a unique family of elements known as the noble gases. Positioned in the far-right column, these elements are distinguished by their exceptional stability and low chemical reactivity. This characteristic inertness stems from their electron configurations, making them particularly interesting in chemistry and various real-world applications. They are all nonmetals and exist as monatomic gases under standard conditions.
The Elements of Group 18
The elements that make up Group 18 include Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn). Oganesson (Og) is also a member, though it is a synthetic, unstable element. Helium is the lightest noble gas. Argon is abundant in Earth’s atmosphere, and Radon is a naturally occurring radioactive gas.
Unique Stability and Reactivity
The defining characteristic of Group 18 elements is their remarkable chemical inertness. This property arises from their electron configuration, specifically their full valence electron shells. With the exception of Helium, which has two valence electrons, all other noble gases possess eight electrons in their outermost shell. This complete outer shell configuration, often referred to as a stable octet, means these atoms have little tendency to gain, lose, or share electrons to form chemical bonds.
Their stability led to them being called “noble” gases, an analogy to the nobility who were considered above interacting with commoners. While generally unreactive, some heavier noble gases, such as Xenon and Krypton, can form compounds under specific, often extreme, conditions, typically with highly electronegative elements like fluorine and oxygen. For example, xenon hexafluoroplatinate was the first noble gas compound discovered in 1962.
Physical Properties and States
All noble gases are gases at standard temperature and pressure. They are colorless, odorless, and tasteless. Their gaseous state is due to very weak intermolecular forces between their atoms, primarily London dispersion forces, which result from temporary fluctuations in electron distribution around the atoms.
Due to these weak forces, noble gases possess very low melting and boiling points; all are below 165 K (-108 °C). For instance, Helium has the lowest boiling point of any known element, at -269 °C. As one moves down Group 18, the atomic size increases, leading to stronger London dispersion forces and consequently higher melting and boiling points.
Practical Uses and Significance
The unique properties of noble gases make them valuable in numerous practical applications.
- Helium: Used to inflate balloons and blimps due to its lightness and non-flammability. It is also employed in cryogenics for cooling superconducting magnets, such as those found in MRI machines, and in specialized gas mixtures for deep-sea diving to prevent decompression sickness.
- Neon: Creates the distinctive bright orange-red glow in “neon” signs when electricity passes through it.
- Argon: Widely used as an inert shielding gas in welding to protect reactive metals from atmospheric gases like oxygen and nitrogen. It also fills incandescent light bulbs to prevent the filament from burning out prematurely and is used in certain fire suppression systems.
- Krypton: Finds application in high-performance light bulbs, including some types of lasers.
- Xenon: Used in high-intensity discharge lamps for car headlights, projection lamps, and photographic flashes, providing very bright light. It has also been explored as a general anesthetic in medicine.
- Radon: A radioactive gas, sees limited use in radiation therapy for certain medical conditions.