The Periodic Table » Group 0: The Noble Gases
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.2.4
- Which elements are the noble gases in Group 0
- Why their outer shell of electrons makes them so unreactive
- How their boiling points change going down the group
- How to predict properties from a trend
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Unlock This CourseMeet the noble gases
Neon signs glow because electricity passes through the gas - neon is so unreactive it just sits in the tube for years
Look at the far right column of the periodic table. This is Group 0. The elements in it are called the noble gases. Going down the group they are helium, neon, argon, krypton, xenon and radon.
All of them are gases at room temperature. They are colourless and have no smell. Most importantly, they are unreactive. Mix a noble gas with almost any other substance and nothing happens. They don't burn, they don't react with metals and they don't react with water.
There is something else unusual about them. Most gaseous elements you meet, like oxygen and chlorine, exist as molecules of two atoms joined together. The noble gases do not easily form molecules. Instead, each particle of a noble gas is a single atom moving around on its own.
Key terms:
- Noble gases: the elements in Group 0 of the periodic table.
- Unreactive: does not easily take part in chemical reactions.
- Stable arrangement of electrons: a full outer shell, so the atom has no tendency to lose, gain or share electrons.
Why are they so unreactive? It's all in the outer shell
Remember, an atom's chemistry depends on its outer electrons. When atoms react, they lose, gain or share electrons, and they end up with a full outer shell.
The noble gases already have one. Their atoms have stable arrangements of electrons, so they have no need to lose, gain or share any. That is why they are unreactive and do not easily form molecules.
○ Helium
Electronic structure: 2. The first shell can only hold two electrons, so it is already full with just two.
◎ Neon
Electronic structure: 2,8. Eight electrons in the outer shell, so the outer shell is stable.
◉ Argon
Electronic structure: 2,8,8. Again, eight electrons in the outer shell.
Krypton, xenon and radon have more shells, but they also have eight electrons in their outer shell. So the rule is simple:
- All noble gases have eight outer electrons, except helium, which has only two.
- Either way, the outer shell is full and stable.
Here is a neat link with ions. When a sodium atom loses one electron, its ion has the structure 2,8. That is the same as neon. When a chlorine atom gains one electron, its ion has the structure 2,8,8, the same as argon. Other elements react and end up with the stable arrangement that noble gas atoms already have.
The boiling point trend
Helium has the lowest boiling point of all the noble gases, about -269 °C - and it's light enough to float these balloons
The noble gases have very low boiling points, which is why they are all gases at room temperature. But the boiling point is not the same for each one. The spec states the trend clearly:
The boiling points of the noble gases increase with increasing relative atomic mass, going down the group.
| Noble gas | Relative atomic mass | Boiling point (°C) |
|---|---|---|
| Helium | 4 | -269 |
| Neon | 20 | -246 |
| Argon | 40 | -186 |
| Krypton | 84 | -153 |
| Xenon | 131 | -108 |
| Radon | 222 | -62 |
Read the table carefully. As you go down, the numbers get less negative. -62 °C is higher than -269 °C. So radon has the highest boiling point and helium has the lowest.
Why does this happen? Going down the group, the atoms get heavier and bigger. The weak forces of attraction between the atoms get stronger. More energy is needed to overcome these forces and turn the liquid into a gas, so the boiling point is higher. You will meet forces between particles again in the lesson on properties of ionic compounds and small molecules.
Predicting properties from a trend
Elements in the same group follow patterns. If you know the trend, you can predict a value you have not been given. Examiners love this skill.
Worked example 1: estimating a missing value
A table gives the boiling points of argon (-186 °C) and xenon (-108 °C), but krypton's is missing. Predict krypton's boiling point.
Step 1: Krypton is between argon and xenon in the group, so its boiling point should be between theirs.
Step 2: A sensible estimate is roughly halfway: (-186 + -108) ÷ 2 = -147 °C.
Answer: about -147 °C. The real value is -153 °C, so the estimate is close. Any value between -186 °C and -108 °C fits the trend.
Worked example 2: predicting a state
Radon is at the bottom of Group 0. Room temperature is about 20 °C. Is radon a gas, liquid or solid at room temperature?
Answer: Its boiling point (-62 °C) is well below 20 °C, so radon is a gas at room temperature, just like the rest of the group.
Worked example 3: predicting reactivity
Would xenon react with sodium?
Answer: No. Xenon is a noble gas with eight electrons in its outer shell. It has a stable arrangement of electrons, so it does not easily react.
Common mistakes
Mixing up negative numbers. Students say the boiling point decreases down the group because the numbers look smaller. But -62 is a higher temperature than -269. The boiling point increases.
Saying all noble gases have eight outer electrons. Helium has only two. Its outer shell is still full.
Writing "full shell" with no link to reactivity. Always finish the chain: full outer shell, so a stable arrangement, so no need to lose, gain or share electrons, so unreactive.
Thinking reactivity changes down the group. All the noble gases are unreactive. It is the boiling point that shows the clear trend.
Exam-style question
Neon and argon are in Group 0 of the periodic table.
(a) Explain why neon is unreactive. Use ideas about electrons. [2 marks]
(b) Neon boils at -246 °C and argon boils at -186 °C. Predict whether krypton's boiling point is higher or lower than argon's. Give a reason. [2 marks]
Model answer
(a) A neon atom has eight electrons in its outer shell (2,8), so its outer shell is full. This is a stable arrangement, so it does not need to lose, gain or share electrons.
(b) Higher. Boiling points increase going down Group 0 as relative atomic mass increases, and krypton is below argon.
Exam tip
For any "explain" question on Group 0, use the words outer shell, full and stable. For a trend question, always say which direction ("going down the group") and link it to relative atomic mass.