Short Answer

Noble gases are the elements of Group 18 in the periodic table, including helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).

• They have completely filled valence shells, making them chemically inert and very stable.
• Noble gases exist as colorless, odorless gases under standard conditions and have low reactivity, although some can form compounds under special conditions.

Detailed Explanation :

General Characteristics of Noble Gases

Noble gases are located in Group 18, also called the inert gases due to their lack of reactivity. Their general electron configuration is ns²np⁶ (except helium, which is 1s²), meaning the outermost shell is completely filled. This full valence shell gives them maximum stability and a low tendency to gain, lose, or share electrons.

Physical Properties

1. State at Room Temperature:
   • All noble gases are gases at room temperature.
2. Color and Odor:
   • Colorless and odorless under normal conditions.
3. Density:
   • Low density for lighter gases like helium and neon; density increases down the group.
4. Melting and Boiling Points:
   • Very low because of weak van der Waals forces.
   • Both increase slightly down the group.
5. Conductivity:
   • Poor conductors of heat and electricity, showing non-metallic nature.

Chemical Properties

1. Extremely Low Reactivity:
   • Full valence shells make them very stable, so they rarely react.
2. Formation of Compounds:
   • Some heavier noble gases, such as xenon and krypton, can form fluorides and oxides under special conditions.
   • Example: Xe + F₂ → XeF₂
3. Oxidizing and Reducing Properties:
   • Helium, neon, argon → almost completely inert.
   • Xenon, krypton → can act as weak oxidizing agents in compounds.
4. Ionization Energy:
   • Very high → difficult to remove electrons.
5. Electron Affinity:
   • Nearly zero → do not readily gain electrons.

Trends in Noble Gases

1. Atomic Size:
   • Increases down the group due to additional electron shells.
2. Density:
   • Increases down the group → radon is the heaviest.
3. Melting and Boiling Points:
   • Slight increase down the group due to stronger van der Waals forces.
4. Reactivity:
   • Decreases with lighter gases (He, Ne, Ar) → almost inert.
   • Slightly higher in heavier gases (Kr, Xe, Rn) under extreme conditions.

Occurrence and Uses

1. Occurrence:
   • Found in small amounts in the atmosphere.
   • Example: Argon makes up ~1% of air.
2. Uses:
   • Helium → balloons, cooling superconducting magnets.
   • Neon → advertising signs.
   • Argon → inert gas in welding.
   • Xenon → high-intensity lamps and anesthesia.
   • Radon → radioactive research, cancer treatment (limited).

Significance

1. Chemical Stability:
   • Full valence shell → explains inert nature.
2. Industrial Applications:
   • Used where non-reactive environment is needed.
3. Periodic Trends:
   • Trends in atomic radius, density, and melting/boiling points are clearly observed in this group.
4. Rare Compounds:
   • Discovery of xenon compounds proves that even noble gases can react under special conditions.

Conclusion

Noble gases are Group 18 elements with fully filled outer shells, making them extremely stable and unreactive. They are colorless, odorless gases with very high ionization energy and negligible electron affinity. Trends in atomic size, density, and boiling/melting points are observed down the group. While generally inert, heavier noble gases like xenon can form compounds under special conditions, which is important in chemistry and industrial applications.