Group 0
What you'll learn
- What the Group 0 elements are and where they are found in the periodic table.
- Why the noble gases are very unreactive.
- Why noble gases usually exist as single atoms, not molecules.
- How to use the trend in boiling points to make predictions down the group.
Where Group 0 fits in the periodic table
The periodic table is arranged in columns called groups. Elements in the same group have similar chemical properties because their atoms have the same number of electrons in their outer shell.
Group 0 is the far-right column of the periodic table. The elements in this group are called the noble gases.
Noble gases
The noble gases are the Group 0 elements. They include helium, neon, argon, krypton, xenon and radon.
You may also see this group called Group 18 on some periodic tables, but for AQA GCSE Chemistry you should recognise the name Group 0.
Electron shells: the key prerequisite
An atom contains electrons arranged in shells around the nucleus. An electron shell is an energy level that can hold electrons.
The outer shell is the furthest shell from the nucleus that contains electrons. This shell is especially important because it controls many chemical properties.
Outer shell
The outer shell is the highest occupied electron shell in an atom. It is the shell involved when atoms react and form bonds.
Chemical reactions usually involve atoms losing, gaining or sharing electrons to get a more stable arrangement.
The noble gases are different because their atoms already have stable outer shells.
Full outer shells and stability
Most noble gases have eight electrons in their outer shell. This is a full outer shell for those atoms.
The exception is helium. Helium only has two electrons, but that is still a full outer shell because the first electron shell can only hold two electrons.
Stable electron arrangements
Group 0 atoms are very unreactive because their outer electron shells are already full. Helium has 2 outer electrons; the other noble gases have 8 outer electrons.
For GCSE, the first few noble gases are especially useful:
| Element | Symbol | Electron arrangement | Outer shell |
|---|---|---|---|
| Helium | He | 2 | Full with 2 electrons |
| Neon | Ne | 2,8 | Full with 8 electrons |
| Argon | Ar | 2,8,8 | Full with 8 electrons |
You do not usually need to draw detailed shell diagrams for the heavier noble gases, but you should know that their outer shells are full.
Using electron structure to explain unreactivity
An atom has the electron arrangement 2,8,8. Explain why it is unreactive.
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The final number in the electron arrangement is 8, so the atom has 8 electrons in its outer shell.
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A full outer shell is a stable arrangement, so the atom does not need to gain, lose or share electrons.
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Because it has little tendency to change its outer shell, the atom is unreactive. The element is argon, a Group 0 noble gas.
Saying noble gases have no electrons
Noble gases are not unreactive because they have “no electrons”. They have electrons, but their outer shells are already full.
Noble gases do not easily form molecules
A molecule is a particle made from two or more atoms chemically bonded together.
For example, oxygen gas is made of oxygen molecules, O₂. Each molecule contains two oxygen atoms bonded together.
Noble gases are different. They usually exist as single atoms because they do not need to share electrons to become stable.
Monatomic
A monatomic gas is made from single atoms rather than molecules. Noble gases are monatomic, for example He(g), Ne(g) and Ar(g).
This is why you should write noble gases as single atoms, such as Ar(g), not as Ar₂(g).
Writing noble gases as diatomic molecules
Group 7 elements such as chlorine exist as Cl₂ molecules. Group 0 noble gases do not: argon is Ar(g), not Ar₂(g).
Why Group 0 elements are gases
The noble gases have very low boiling points, so they are gases at room temperature.
Boiling point
The boiling point is the temperature at which a liquid changes into a gas.
The atoms in noble gases are not joined by covalent bonds. There are only weak forces between the atoms. These weak forces do not need much energy to overcome, so the boiling points are low.
Trend in boiling points down Group 0
As you go down Group 0, the relative atomic mass of the elements increases.
Relative atomic mass
The relative atomic mass, often shown as Ar, compares the mass of an atom with the mass of carbon-12. It has no units.
The boiling points of the noble gases increase as relative atomic mass increases.
That means boiling points get higher going down the group:
- helium has the lowest boiling point
- neon has a higher boiling point than helium
- argon has a higher boiling point than neon
- krypton, xenon and radon have higher boiling points still
Boiling point trend
Going down Group 0: relative atomic mass increases, atoms become larger, the forces between atoms become stronger, and boiling points increase.
The boiling points are often negative numbers, so be careful. A temperature of -186 °C is higher than -246 °C because it is less cold.
Misreading negative boiling points
For negative temperatures, the number closer to zero is the higher temperature. So -153 °C is higher than -186 °C.
Explaining the boiling point trend
As noble gas atoms get larger down the group, they have more electrons.
This makes the weak forces between atoms stronger. More energy is needed to separate the atoms when the liquid boils, so the boiling point increases.
You do not need A-Level explanations such as named intermolecular forces in detail here. For GCSE, “stronger forces between atoms, so more energy is needed to boil” is enough.
Predicting a boiling point from the trend
A table gives these boiling points for Group 0 elements:
| Element | Boiling point |
|---|---|
| Argon | -186 °C |
| Xenon | -108 °C |
Krypton is between argon and xenon in Group 0. Predict whether krypton’s boiling point is likely to be -250 °C, -153 °C or 20 °C.
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Krypton is below argon, so its boiling point should be higher than -186 °C.
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Krypton is above xenon, so its boiling point should be lower than -108 °C.
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The only value between -186 °C and -108 °C is -153 °C, so that is the best prediction.
Predicting properties from Group 0 trends
In exams, you may be given a table or graph and asked to predict a missing value.
You are not expected to memorise exact boiling points. Instead, you need to use the trend:
- down the group, relative atomic mass increases
- down the group, boiling point increases
- noble gases remain very unreactive because their outer shells are full
Trend prediction shortcut
If an element is between two others in Group 0, its boiling point should usually be between their boiling points too.
For example, if argon has a lower boiling point than krypton, and xenon is below krypton, then xenon should have an even higher boiling point.
The big picture
Group 0 is a neat example of how the periodic table links structure to properties.
The structure is the electron arrangement: noble gas atoms have full outer shells.
The properties follow from that structure: they are very unreactive, do not easily form molecules, and exist as gases with low boiling points.
The trend down the group is physical rather than chemical: boiling points increase because the atoms get larger and the forces between atoms get stronger.
In the exam
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When asked why noble gases are unreactive, link your answer to a full outer shell of electrons, not just “they are stable”.
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Remember the helium exception: helium has 2 electrons in its outer shell, while the other noble gases have 8.
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For boiling point predictions, check the order down the group and be careful with negative temperatures.
Check yourself
- Why does helium count as having a full outer shell even though it only has two electrons?
- Why do noble gases usually exist as single atoms rather than molecules?
- If an unknown noble gas is below argon in Group 0, what should happen to its boiling point compared with argon?
