The noble gases are the elements in Group 18 of the periodic table: helium, neon, argon, krypton, xenon, radon, and oganesson. They are famous for being very unreactive because their outer electron shells are filled or effectively complete. This stability makes them useful whenever chemists and engineers need a gas that will not easily burn, corrode, or react.
Their bright colors in electric discharge tubes also make them important in lighting and scientific instruments.
Noble gases have very high ionization energies and very low tendencies to gain or share electrons, so most of them exist as single atoms rather than molecules. Helium has 2 valence electrons, while the others generally have 8 valence electrons in their outer shell. Although they are often called inert gases, heavier noble gases such as xenon and krypton can form compounds under special conditions, especially with highly electronegative elements like fluorine and oxygen.
Their uses include balloons, cryogenics, neon signs, welding atmospheres, light bulbs, lasers, and radiation-related applications.
Key Facts
- Group 18 elements are He, Ne, Ar, Kr, Xe, Rn, and Og.
- Helium has a filled first shell with 2 valence electrons: 1s2.
- Most other noble gases have a full valence shell with 8 valence electrons: ns2 np6.
- Low reactivity comes from high ionization energy and a stable outer electron configuration.
- Ideal gas law for many noble gas calculations: PV = nRT.
- Examples of noble-gas compounds include XeF2, XeF4, XeF6, and KrF2.
Vocabulary
- Noble gas
- A Group 18 element that is usually very unreactive because it has a filled outer electron shell.
- Valence shell
- The outermost occupied electron shell of an atom, which controls most chemical bonding behavior.
- Ionization energy
- The energy required to remove an electron from an atom or ion in the gas phase.
- Inert atmosphere
- A nonreactive gas environment used to protect materials from reacting with oxygen, water, or other chemicals.
- Discharge tube
- A sealed tube containing low-pressure gas that glows when an electric current passes through it.
Common Mistakes to Avoid
- Saying noble gases never react, because this ignores rare compounds such as xenon fluorides that form under special conditions.
- Giving helium 8 valence electrons, because helium only has the first shell and is stable with 2 electrons.
- Calling all glowing signs neon signs, because different noble gases produce different colors and many signs use argon, krypton, xenon, or mixtures.
- Assuming low reactivity means low usefulness, because noble gases are valuable precisely because they provide stable, nonreactive environments.
Practice Questions
- 1 A balloon contains 0.50 mol of helium gas at 300 K and 1.00 atm. Using PV = nRT with R = 0.0821 L atm mol^-1 K^-1, what is the volume of the balloon?
- 2 Argon makes up about 0.934% of dry air by volume. In a 1000 L sample of dry air, what volume of argon is present?
- 3 Explain why argon is often used inside light bulbs or as a shielding gas in welding instead of oxygen or nitrogen.