Important Groups
3. Group 0/18: Noble Gases
Learning outcomes
- I can identify the noble gases on the periodic table.
- I can describe the physical properties of noble gases.
- I can explain why noble gases are generally unreactive.
- I can relate noble gas stability to electron arrangements.
- I can describe practical applications of noble gases.
What Are the Noble Gases?
The noble gases are the elements found in Group 18 of the modern periodic table.
In some school systems and older textbooks, this group is called Group 0.
The noble gases are:
- helium (He)
- neon (Ne)
- argon (Ar)
- krypton (Kr)
- xenon (Xe)
- radon (Rn)
- oganesson (Og)
The first six are the noble gases most commonly discussed in introductory chemistry.
Noble gases are unusual because they are extremely unreactive compared with most other elements.
Finding the Noble Gases
Group 18 is the vertical column on the far-right side of the periodic table.
Moving down the group:
He → Ne → Ar → Kr → Xe → Rn → Og
Their position at the end of each period is closely connected to their electron arrangements and chemical stability.
Electron Arrangements of Noble Gases
The most important feature of noble gases is their complete outer electron shell.
Helium:
2
Neon:
2,8
Argon:
2,8,8
Neon and the other main-group noble gases have eight valence electrons in their outer shell.
Helium is the exception.
Its first shell can hold only two electrons, so helium is stable with:
2 electrons
A Full Outer Electron Shell
Atoms often react because gaining, losing, or sharing electrons can produce a more stable electron arrangement.
Noble gases already have very stable outer electron arrangements.
They therefore have little tendency to:
- gain electrons
- lose electrons
- share electrons
This explains their very low chemical reactivity.
Why Are Noble Gases Unreactive?
Consider neon.
Its electron arrangement is:
2,8
Its outer shell is already full.
Neon does not need to gain an electron because the outer shell is complete.
It also does not readily lose electrons because removing electrons from this stable arrangement requires considerable energy.
Therefore, neon usually remains as individual:
Ne atoms
rather than forming ions or molecules.
Stability and Electron Arrangements
The stability of noble gases helps explain why many other elements react.
For example:
Sodium:
2,8,1
can lose one electron to form:
Na⁺ → 2,8
Chlorine:
2,8,7
can gain one electron to form:
Cl⁻ → 2,8,8
Both ions then have electron arrangements corresponding to nearby noble gases.
This is one reason noble-gas configurations are so important in understanding chemical bonding.
The Octet Rule
A useful introductory model is the octet rule.
Many main-group atoms tend to gain, lose, or share electrons in ways that give them eight electrons in their outer shell.
The rule helps explain many simple examples of:
- ionic bonding
- covalent bonding
- ion formation
However, it is a useful model rather than an absolute rule. There are many important exceptions in chemistry.
Helium and the Duet Rule
Helium does not have eight valence electrons.
It has only:
2 electrons
Why is it stable?
The first electron shell can hold a maximum of only two electrons.
Therefore, helium's first shell is completely filled.
This is sometimes described using the duet rule.
Helium is therefore chemically stable even though it does not have an octet.
Noble Gases Are Monatomic
Many elements exist naturally as molecules.
For example:
Hydrogen = H₂
Oxygen = O₂
Chlorine = Cl₂
Noble gases are different.
They normally exist as individual atoms.
This is described as:
monatomic
For example:
He
Ne
Ar
rather than He₂, Ne₂, or Ar₂.
Why Are Noble Gases Monatomic?
A covalent bond forms when atoms share electrons.
Noble gas atoms already have stable outer electron arrangements.
They therefore have little tendency to form ordinary covalent bonds with another identical atom.
As a result, ordinary samples of noble gases consist mainly of separate atoms.
Physical Properties of Noble Gases
Noble gases share several physical properties.
They are generally:
- gases at room temperature
- colourless
- odourless
- monatomic
- non-flammable
- low-density compared with liquids and solids
- relatively low in melting and boiling points
They are also poor electrical conductors under ordinary conditions.
Noble Gases Are Colourless
The gases themselves are colourless under ordinary conditions.
This is important because noble gases are often associated with colourful lights.
The colours seen in gas-discharge tubes occur when electrical energy excites electrons in the atoms.
When the electrons return to lower energy levels, they emit light at characteristic wavelengths.
The gas itself is not naturally glowing or coloured.
Melting and Boiling Points
Noble gases have very low melting and boiling points.
However, both generally increase down Group 18.
So:
He → Ne → Ar → Kr → Xe → Rn
shows an overall increase in melting and boiling temperatures.
This means helium remains a gas at much lower temperatures than xenon.
Why Boiling Points Increase Down the Group
Moving down Group 18:
- atoms become larger
- the number of electrons increases
- electron clouds become more polarizable
- London dispersion forces between atoms become stronger
Therefore, more energy is needed to separate the atoms.
As a result:
boiling point generally increases down the group.
Density Down Group 18
Density also generally increases down Group 18.
Helium has a very low density.
Heavier noble gases such as krypton and xenon are considerably denser.
The increase occurs largely because the atoms become progressively more massive down the group.
Are Noble Gases Completely Inert?
Noble gases were once often described as inert gases, meaning that they did not react chemically.
Today, we know this description is not completely accurate.
The heavier noble gases, especially xenon, can form compounds under suitable conditions.
Examples include compounds containing xenon and fluorine.
Therefore, it is more accurate to say:
noble gases are generally very unreactive
rather than:
noble gases never react.
Why Heavier Noble Gases Can React
Moving down Group 18:
- atomic radius increases
- shielding increases
- outer electrons become farther from the nucleus
- first ionization energy generally decreases
This means the outer electrons of heavier noble gases are held somewhat less strongly than those of lighter noble gases.
As a result, elements such as xenon can participate in some reactions under suitable conditions.
Noble Gases and Ionization Energy
Noble gases have relatively high first ionization energies because of their stable electron arrangements.
Removing an electron from a stable noble-gas configuration generally requires substantial energy.
This contributes to their low chemical reactivity.
Noble Gases Do Not Normally Form Ions
Group 1 metals commonly form:
+1 ions
Group 17 halogens commonly form:
−1 ions
Noble gases generally do neither.
They already have stable outer electron arrangements.
Therefore, under ordinary introductory chemistry conditions, noble gases are not usually assigned common ionic charges.
Noble Gas Stability and Chemical Bonding
The stable electron arrangements of noble gases help us understand why other atoms form chemical bonds.
Consider magnesium:
2,8,2
Magnesium can lose two electrons:
Mg → Mg²⁺ + 2e⁻
The Mg²⁺ ion then has:
2,8
This is the same electron arrangement as neon.
Another Example: Oxygen
Oxygen has:
2,6
It can gain two electrons:
O + 2e⁻ → O²⁻
The oxide ion then has:
2,8
Again, the resulting ion has the same electron arrangement as neon.
This connection helps explain many common ionic charges.
Noble Gases and Lighting
One of the best-known applications of noble gases is in lighting.
When electricity passes through a low-pressure noble gas, atoms can absorb energy.
Their electrons move temporarily to higher energy levels.
When the electrons return to lower energy levels, they release energy as light.
Different gases produce different characteristic emission spectra.
Neon Signs
Neon is famous for its use in illuminated signs.
Electrical energy excites the neon atoms.
When their electrons return to lower energy states, neon produces a characteristic red-orange glow.
Many signs casually called "neon signs" actually use other gases or lighting technologies to produce different colours.
Argon in Light Bulbs
Argon is commonly used where an unreactive atmosphere is useful.
One traditional application is inside certain light bulbs.
The hot filament would react much more readily if surrounded by oxygen.
Argon provides an atmosphere that is much less chemically reactive.
This helps protect the filament.
Argon in Welding
Argon is also widely used as a shielding gas in welding.
During welding, very hot metals can react with gases in the surrounding air.
Argon can surround the welding area and reduce contact with reactive atmospheric gases.
Its low chemical reactivity makes it useful for this purpose.
Helium in Balloons
Helium has a very low density.
It is less dense than air.
Therefore, helium-filled balloons experience a buoyant force that can make them rise.
Helium also has an important advantage over hydrogen:
helium is non-flammable.
This makes it safer for many applications where a low-density gas is required.
Helium in Scientific and Medical Technology
Liquid helium can reach extremely low temperatures.
It is therefore important as a cryogenic coolant.
One important application is cooling certain superconducting magnets, including magnets used in some MRI systems and scientific equipment.
Helium's exceptionally low boiling point makes it particularly useful for very-low-temperature applications.
Helium in Leak Detection
Helium atoms are small, and helium is chemically unreactive.
These properties make helium useful for detecting very small leaks in sealed systems.
A component can be exposed to helium while sensitive instruments detect whether helium passes through tiny openings.
This technique is used in some:
- vacuum systems
- scientific instruments
- industrial equipment
- aerospace systems
Xenon in Lighting
Xenon is used in several specialized lighting technologies.
Xenon lamps can produce intense light and have been used in applications such as:
- flash lamps
- projection systems
- specialized high-intensity lighting
Xenon's electronic structure produces characteristic light when excited.
Krypton in Lighting
Krypton can also be used in specialized lamps and gas-discharge devices.
Like other noble gases, its low reactivity makes it useful where a gas is needed that will not readily react with surrounding materials.
Different noble gases produce different emission spectra, which can also be used to identify them.
Noble Gases and Emission Spectra
Each element has a unique arrangement of electron energy levels.
When noble-gas atoms absorb electrical energy, their electrons can move to higher energy levels.
When they return:
energy is emitted as light.
The wavelengths produced form a characteristic emission spectrum.
This acts somewhat like an atomic fingerprint.
Noble Gases in Earth's Atmosphere
Several noble gases occur naturally in Earth's atmosphere.
Argon is by far the most abundant noble gas in ordinary air.
Other noble gases, including neon, helium, krypton, and xenon, are present in much smaller amounts.
Their low reactivity means they can remain in the atmosphere without readily forming compounds with other substances.
Radon
Radon is different from the lighter noble gases because it is radioactive.
It is produced naturally through radioactive decay processes involving elements in rocks and soil.
Radon can accumulate in enclosed spaces under some geological and building conditions.
Although it is chemically unreactive, its radioactivity means that it presents a different type of hazard from ordinary chemical reactivity.
What About Oganesson?
Oganesson is the element at the bottom of Group 18.
Unlike helium, neon, and argon, it is a synthetic, extremely short-lived radioactive element.
Only very small numbers of oganesson atoms have been produced.
Because of its extreme mass and unusual electron behaviour, scientists expect some of its properties to differ from the simpler trends established by the lighter noble gases.
For introductory chemistry, most Group 18 trends are therefore best understood using:
He, Ne, Ar, Kr, Xe, and Rn.
Comparing Group 1, Group 17, and Group 18
These three groups provide a useful comparison.
Group 1
One valence electron.
Usually loses one electron.
Forms +1 ions.
Highly reactive.
Group 17
Seven valence electrons.
Usually gains one electron.
Forms −1 ions.
Reactive non-metals.
Group 18
Full outer electron shell.
Usually neither gains nor loses electrons.
Generally very unreactive.
This comparison demonstrates how strongly chemical behaviour is connected to valence electrons.
Worked Example 1
Why is neon generally unreactive?
Neon has the electron arrangement:
2,8
Its outer electron shell is full.
Therefore, neon has little tendency to gain, lose, or share electrons.
As a result:
neon is chemically very unreactive.
Worked Example 2
Why does sodium react readily while neon does not?
Sodium:
2,8,1
Neon:
2,8
Sodium can achieve a more stable arrangement by losing one electron.
Neon already has a stable full outer shell.
Therefore:
sodium is reactive, while neon is very unreactive.
Worked Example 3
Why is helium used in balloons instead of hydrogen in many situations?
Both helium and hydrogen have very low densities.
However:
hydrogen is flammable
while:
helium is non-flammable and chemically very unreactive.
Therefore, helium can provide an important safety advantage.
Worked Example 4
Why is argon useful during welding?
Hot metals can react with substances in the air.
Argon is chemically very unreactive.
It can therefore provide a protective atmosphere around the welding region and reduce unwanted reactions.
Worked Example 5
An unknown element is located in Group 18.
Predict two of its likely properties.
A reasonable prediction is that it:
- has a full outer electron shell
- is generally chemically unreactive
- is likely to exist as individual atoms
- does not readily form common ions
This demonstrates how periodic position can be used to predict properties.
Explaining Properties from Electron Arrangement
When explaining noble gases, a useful reasoning chain is:
Full outer electron shell
↓
Stable electron arrangement
↓
Little tendency to gain or lose electrons
↓
Little tendency to form ordinary chemical bonds
↓
Very low chemical reactivity
This is the central idea behind Group 18 chemistry.
Practical Applications and Properties
The uses of noble gases are closely connected to their properties.
Helium
Low density + non-flammable → balloons.
Very low boiling point → cryogenic cooling.
Neon
Characteristic light emission → illuminated signs.
Argon
Low reactivity → protective atmospheres in lamps and welding.
Krypton
Low reactivity + characteristic emission → specialized lighting.
Xenon
Characteristic emission + useful physical properties → specialized lamps and other technologies.
Understanding the property allows us to explain the application, rather than simply memorizing a list of uses.
Common Mistakes
Mistake 1: Saying noble gases have no electrons in their outer shell
They have a full outer shell.
Mistake 2: Saying every noble gas has eight valence electrons
Helium has only two electrons.
Its first shell is full with:
2 electrons.
Mistake 3: Saying noble gases form diatomic molecules
Noble gases are normally:
monatomic.
For example:
He, Ne, Ar
not He₂, Ne₂, or Ar₂.
Mistake 4: Saying noble gases are completely unable to react
It is more accurate to say:
noble gases are generally very unreactive.
Some heavier noble gases can form compounds under suitable conditions.
Mistake 5: Saying neon gas is naturally red
Neon gas is colourless under ordinary conditions.
The familiar red-orange light appears when neon atoms are electrically excited.
Mistake 6: Confusing chemical stability with radioactivity
Radon is chemically unreactive but radioactive.
Chemical reactivity and nuclear stability are different properties.
Did You Know?
The name noble gas reflects the idea that these elements generally avoid reacting with other elements.
Early chemists initially believed they were completely incapable of forming compounds.
Later discoveries showed that some heavier noble gases, particularly xenon, can form compounds.
This is a useful example of how scientific explanations can change as new evidence becomes available.
Key Terms
Noble gas: An element found in Group 18 of the periodic table.
Valence electron: An electron in the outermost occupied shell.
Full outer shell: An outer electron shell containing its stable complement of electrons.
Monatomic: Existing as individual atoms rather than bonded molecules.
Unreactive: Having a very low tendency to undergo chemical reactions.
Octet rule: A useful model in which many main-group atoms gain, lose, or share electrons to obtain eight outer-shell electrons.
Duet rule: The stable two-electron arrangement of the first electron shell.
Ionization energy: The energy required to remove an electron from an isolated gaseous atom.
Emission spectrum: The characteristic wavelengths of light emitted by excited atoms.
Shielding gas: An unreactive gas used to protect a material from unwanted chemical reactions.
Cryogenic: Relating to extremely low temperatures.
Key Patterns
Group 18 elements:
He, Ne, Ar, Kr, Xe, Rn, Og
Outer electron arrangements:
He → full first shell with 2 electrons
Ne and other typical main-group noble gases → full outer shell
Physical state at room temperature:
gases
Particle structure:
monatomic
Chemical behaviour:
generally very unreactive
Down the group:
atomic size generally increases
density generally increases
melting and boiling points generally increase
first ionization energy generally decreases
Key Takeaways
- Noble gases occupy Group 18, also called Group 0 in some systems.
- The noble gases include helium, neon, argon, krypton, xenon, radon, and oganesson.
- Noble gases are found on the far-right side of the periodic table.
- They are generally colourless, odourless gases at room temperature.
- Noble gases normally exist as individual atoms, making them monatomic.
- Their low reactivity is explained by their stable, complete outer electron shells.
- Helium has a full first shell containing two electrons.
- Neon and the other common main-group noble gases have complete outer valence shells.
- Noble gases therefore have little tendency to gain, lose, or share electrons.
- They generally do not form common positive or negative ions.
- Their stable electron arrangements help explain why other atoms gain, lose, or share electrons during chemical bonding.
- Melting and boiling points generally increase down Group 18 because intermolecular attractions become stronger as atoms become larger and more polarizable.
- Density generally increases down the group.
- Noble gases are better described as very unreactive rather than completely inert, because some heavier noble gases can form compounds.
- Helium is useful because of its low density, non-flammability, and extremely low boiling point.
- Neon is well known for its characteristic light emission in electrical discharge tubes.
- Argon is useful when an unreactive atmosphere is required, including some welding and lighting applications.
- Krypton and xenon have specialized lighting applications.
- Noble-gas applications can be explained by connecting physical or chemical properties to their practical uses.
- The central relationship is:
electron arrangement → stability → low tendency to gain or lose electrons → low chemical reactivity.