Group 18: noble gases and group comparisons
Key idea: Noble gases have full outer electron shells and are very unreactive. Group 1 atoms lose an electron in reactions; Group 17 atoms gain one.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Explain low reactivity using full outer shells.
- Compare neon and chlorine outer shells.
- Explain why their electron arrangements give different reactivities.
- Identify full shells and predict magnesium’s ion charge.
- Explain why two outer electrons give different behaviour.
- Compare electron loss, gain and already-full shells.
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ContinueNeon and chlorine are both gases, but their reactivities differ greatly. Explain why a full outer shell matters, then compare the electron-loss, electron-gain and unreactive patterns across Groups 1, 17 and 18.
Use the electron arrangements from Periodic Table position and the trends from Group 1 and Group 17.
Group 18: the noble gases
Helium, neon and argon are the first three Group 18 elements. They are colourless gases, and they are very unreactive. The reason is their electron arrangement.
| Element | Proton number | Electron arrangement | Outer shell |
|---|---|---|---|
| helium | 2 | 2 | first shell full with 2 electrons |
| neon | 10 | 2,8 | 8 outer electrons |
| argon | 18 | 2,8,8 | 8 outer electrons |
For the simple substances studied here, losing, gaining or sharing electrons often gives atoms a full outer shell. Noble gas atoms already have one, so they have almost no tendency to lose, gain or share electrons. They do not even bond to each other: a noble gas is made of single atoms.
Here is evidence of how unreactive they are. Argon makes up almost 1% of air, yet it was not identified until 1894. It was found by removing the oxygen and nitrogen from a sample of air with substances that react with them. A small volume of gas was always left over, because argon did not react with anything used.
Very unreactive does not mean that no compound can ever form. Chemists have made compounds of heavier noble gases, such as xenon, under suitable conditions. Helium, neon and argon form no compounds in ordinary conditions.
Guided practice 1
Why is neon unreactive?
Problem
A learner says neon is unreactive “because it is a gas”. Chlorine is also a gas. First compare their outer shells. In the next activity, use the electron arrangements to give the complete reason for their different reactivities.
Compare the two outer shells
Hints
Hint 1
Test the claim
If being a gas made an element unreactive, chlorine would be unreactive too.
Hint 2
Why atoms react
Atoms react by losing, gaining or sharing electrons to reach a full outer shell.
Show solution step by step
Reject the state explanation
Method
Being a gas does not decide reactivity.Reason
Chlorine is a gas and is very reactive.Working
Answer
Same state, opposite behaviour.Use the electron arrangement
Method
Neon’s outer shell is already full.Reason
It has no tendency to lose, gain or share electrons, so it is very unreactive.
Working
Answer
Neon 2,8: full outer shell → very unreactive. Chlorine 2,8,7: one short → reactive.
Quick check
For helium, the full shell holds two electrons, not eight. The rule is “full outer shell”, not “eight electrons”.
Check your understanding 1 Give the reason beyond the choices
Neon (2,8) and chlorine (2,8,7) are both gases. Explain why neon is very unreactive while chlorine reacts readily. Work on paper or in the optional notes.
Show answer
Neon already has a full outer shell, so it has almost no tendency to lose, gain or share electrons under ordinary conditions. Chlorine is one electron short of a full outer shell and can gain or share an electron in reactions. Being a gas cannot be the explanation: the gases have different reactivities because their electron arrangements differ.
Check your understanding 2 Two outer electrons are not always a full shell
Helium has arrangement 2; magnesium has 2,8,2. State whether each outer shell is full and predict the signed charge on the usual magnesium ion.
Show answer
Helium’s first shell is full with two electrons. Magnesium’s third shell is not full; losing its two outer electrons gives Mg²⁺ with arrangement 2,8. The next activity asks for the complete comparison.
Check your understanding 3 Two outer electrons, different behaviour
Helium has electron arrangement 2; magnesium has 2,8,2. Both have two outer electrons. Should they have the same tendency to form ions? Explain using the shells rather than the outer-electron count alone.
Write your explanation on paper or in the optional notes and compare the shells, electron change and ion charge.
Show answer
No. Helium’s first and only shell is already full with two electrons, so it has almost no tendency to form ions. Magnesium’s third shell is not full: it can lose its two outer electrons to give Mg²⁺, with arrangement 2,8. The same outer-electron count does not mean the shells are equally full.
Three groups, three patterns
| Group | Reacts by | Reactivity down the group | Reason |
|---|---|---|---|
| 1 | losing 1 electron | increases | outer electron farther away and more shielded, so lost more easily |
| 17 | gaining 1 electron | decreases | incoming electron farther away and more shielded, so gained less easily |
| 18 | not losing, gaining or sharing | very unreactive throughout | outer shell already full |
Do not learn one rule such as “reactivity increases down a group” and apply it to every group. Work out whether the atoms lose or gain electrons first.
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Syllabus and review details
- SEC G2 Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 25–33
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