Group 18 elements (the noble gases) have a full outermost shell, a duplet of 2 for helium and an octet of 8 for the others, which is a stable arrangement. They therefore have no tendency to gain, lose or share electrons, so they are chemically inert, exist as single atoms, and are used where an unreactive gas is needed.
This page covers a single Form 4 content standard: Group 18, the noble gases. It is a short, high-value topic because the whole of their behaviour comes from one idea, a full, stable outer shell, and the exam almost always asks you to explain their lack of reactivity using electron arrangement. Learn that explanation properly, together with a few properties and uses, and this standard is secure.
What the noble gases are
Group 18 contains helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn). They are also called the inert gases because they are extremely unreactive, they take almost no part in chemical reactions.
The reason lies in their electron arrangement. Each noble gas has a full outermost shell:
- Helium is 2, its only shell (the first) is full with 2 electrons, a stable duplet.
- Neon is 2.8, its outer shell is full with 8 electrons, a stable octet.
- Argon is 2.8.8, its outer shell is also full with 8 electrons.
A full outer shell is a stable arrangement, so a noble gas atom has no tendency to gain, lose or share electrons. That is exactly why it does not form ions or bonds under normal conditions, and why the other elements are trying to reach a noble-gas arrangement when they react.
Physical properties
The noble gases share these physical properties:
- They are colourless gases at room temperature.
- They exist as single atoms (they are monatomic), unlike most other gases which are molecules.
- They have very low melting and boiling points.
- Going down the group, the boiling point increases and the density increases.
Uses of the noble gases
Their uses come directly from being unreactive, plus one or two special physical properties:
- Helium is used to fill balloons and airships because it has a very low density and, unlike hydrogen, does not burn. It is also used in breathing-gas mixtures for deep-sea divers.
- Neon is used in advertising lights and display signs, where it glows brightly when a current passes through it.
- Argon is used to fill light bulbs and to provide an inert atmosphere in welding, because it will not react with the hot metal filament or weld.
The common thread is that a job needing a gas that will not react is a job for a noble gas.
Worked example
Question. Argon has the electron arrangement 2.8.8. (a) State the number of valence electrons in an argon atom. (b) Explain, in terms of its electron arrangement, why argon is chemically unreactive. (c) State one use of argon that depends on this property.
Step 1, Read the valence electrons. The outermost shell of 2.8.8 contains 8 electrons, so argon has 8 valence electrons.
Step 2, Explain the inertness. Eight electrons in the outer shell is a full, stable octet. Because the shell is already full, argon has no tendency to gain, lose or share electrons, so it does not react and does not form compounds under normal conditions.
Step 3, Give a use. Argon is used to fill light bulbs (or to provide an inert atmosphere in welding), because it will not react with the hot filament.
Answer. (a) 8 valence electrons. (b) Its outer shell is a full, stable octet, so it has no tendency to gain, lose or share electrons and is therefore inert. (c) Filling light bulbs.
Practice question
Question. (a) Write the electron arrangement of neon (proton number 10). (b) Helium has only 2 electrons, not 8 in its outer shell. Explain why helium is still unreactive.
Answer. (a) Neon has 10 electrons, arranged as 2.8, a full outer shell of 8. (b) Helium is 2: its first (and only) shell is full with 2 electrons, a stable duplet. A full outer shell, whether a duplet of 2 or an octet of 8, is stable, so helium has no tendency to gain, lose or share electrons and is unreactive.
Exam tip
The one mistake to avoid is saying every stable atom must have 8 outer electrons. Helium is stable with a duplet of 2, because the first shell is full at 2. So state the rule as “a full outer shell is stable” and mention the duplet for helium. When you explain inertness, the marking scheme wants the phrase “no tendency to gain, lose or share electrons”, a full-shell answer without this is often not enough. Finally, remember that noble gases are monatomic (single atoms), which is another consequence of them not needing to bond.
Where this fits
This standard rounds off the group chemistry in the The Periodic Table of Elements chapter, and the “stable full shell” idea is the direct link to the next chapter on the Chemical Bond, where atoms gain, lose or share electrons to reach a noble-gas arrangement. Revise the properties and uses with the revision notes and check your explanations with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure your inertness explanation is complete and that you never forget the helium duplet, which is the detail most often missed on this standard across SPM Chemistry.
Quick recap
- Group 18 noble gases have a full outer shell (duplet of 2 for helium, octet of 8 for the rest).
- They are inert because they have no tendency to gain, lose or share electrons.
- They are colourless, monatomic gases with very low boiling points.
- Uses (helium in balloons, neon in signs, argon in bulbs) all depend on being unreactive.
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