- The noble gases are the elements in Group 18 of the periodic table (sometimes called Group 0 in older notation).
- They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).
- They are often described as “inert” or unreactive because they rarely form compounds.
- To understand why, we need to look at their electron configurations.
- Noble gases have full outer (valence) electron shells.
- A full valence shell is a very stable arrangement of electrons.
- Because their outer shells are already full, noble gases do not need to gain, lose or share electrons.
- As a result, they very rarely react with other elements.
- A full valence shell is the key to stability in atomic structure.
- For most atoms, this means 8 electrons in the outer shell (the “octet rule”).
- For helium, a full shell means 2 electrons (the “duet rule”) because it only has the first shell.
| Element | Electron Configuration | Stability |
|---|
| Helium | 1s² | Full 1st shell |
| Neon | 1s² 2s² 2p⁶ | Full 2nd shell |
| Argon | 1s² 2s² 2p⁶ 3s² 3p⁶ | Full 3rd shell |
These configurations all show completely filled outer shells, which explains why these atoms are so unreactive under normal conditions.
Helium is the simplest noble gas:
- Electron configuration: 1s²
- It has only one shell, and that shell is fully occupied with 2 electrons.
- This full first shell makes helium very stable and chemically unreactive.
- We often say noble gases are inert, but strictly they are very unreactive, not completely impossible to react.
- Under extreme conditions, some noble gases (especially xenon and krypton) do form compounds, such as xenon fluorides.
- For MYP level, however, it is enough to remember that noble gases are chemically very stable and rarely form compounds.
The unique electron configurations of noble gases lead to several important properties:
- Very low reactivity:
- As discussed earlier, noble gases have full valence shells, which makes them very stable.
- They do not need to gain, lose or share electrons, so they rarely form compounds.
- Monatomic and non-polar:
- Noble gases exist as single atoms (monatomic gases), such as He(g), Ne(g) and Ar(g).
- Because they are not bonded to other atoms, they are non-polar and interact only weakly with each other.
- Low melting and boiling points:
- There are only weak van der Waals (dispersion) forces between noble gas atoms.
- As a result, they have low melting and boiling points and are gases at room temperature.
Because of their inertness and physical properties, noble gases are very useful in many practical applications.
- So-called “neon lights” can contain different noble gases, not just neon.
- When an electric current passes through the gas at low pressure, the atoms are excited and then emit light as they return to lower energy levels.
- Different noble gases produce different colours.
- Neon (Ne) → emits a red–orange glow.
- Argon (Ar) → often gives a blue–violet light and is used in fluorescent tubes.
- Krypton (Kr) and xenon (Xe) → used in high-intensity lamps, such as camera flashes and car headlights.
- Argon is commonly used as a shielding gas in welding.
- Its inert nature prevents the hot metal from reacting with oxygen or nitrogen in the air.
- This protects the weld from contamination and improves the quality of the joint.
- Noble gases such as argon and helium can be used to displace oxygen in food packaging.
- Less oxygen means less oxidation and slower growth of many microorganisms.
- This helps to extend the shelf life of perishable foods.
- In industry, nitrogen is also commonly used for this purpose.
Even though are noble gases unreactive, monitoring their presence in the atmosphere is important.
Understanding noble gases helps scientists study the history of Earth’s atmosphere and provides clues about the atmospheres of other planets as well.
- Argon (Ar) makes up about 0.93% of Earth’s atmosphere.
- It is the third most abundant gas, after nitrogen (N₂) and oxygen (O₂).
- Scientists use argon isotopes (for example in K–Ar dating) to estimate the age of rocks and minerals.
- Helium (He) is used in many applications:
- Filling balloons and airships (light and non-flammable)
- Cooling superconducting magnets in MRI scanners and other scientific instruments
- Helium is produced in limited amounts (e.g. from radioactive decay in rocks and extracted from natural gas) and can be lost to space.
- Therefore, helium is a valuable resource that needs careful management.
Because noble gases are chemically inert, they can act as excellent tracers:
- They can be used to track the movement of air masses in the atmosphere.
- In ice cores and groundwater, dissolved noble gases help scientists estimate the age of the samples and the conditions under which they formed.
- This information contributes to our understanding of climate change and helps in predicting future environmental trends.
- The non-reactivity of noble gases, due to their full electron shells, makes them stable and safe for applications where reactivity would be dangerous or undesirable.
- Because they rarely react, noble gases are not used as reactants in processes that require chemical change, but instead are used as protective atmospheres, coolants or tracers.
Noble gases are often used as examples of ideal gases because:
- They are monatomic.
- They have weak intermolecular forces.
This makes them useful for understanding and applying the ideal gas equation later in IB Chemistry.
- Why are noble gases unreactive?
- Describe one use of argon in everyday life.
- How do noble gases contribute to scientific research and our understanding of Earth’s history or climate?