# T.1(a).7–8 Noble Gases · Reactivity of Groups

F.3 Chemistry · Topic I(a) Nature of Matter · Lesson 4

Why noble gases are stable, and how reactivity changes down Groups I, II and VII.

## Key points

- **Noble gases ／ 貴氣體** (Group 0) are stable: each atom has **8 electrons** in its outermost shell, except **helium**, which has **2**.
- **Octet rule ／ 八隅體規則**: atoms become stable by having 8 electrons in their outermost shells (an **octet structure**), or 2 for some smaller atoms (a **duplet structure**).
- **Alkali metals ／ 鹼金屬** (Group I) are soft metals with low densities, stored under **paraffin oil**. They react with water, giving off **hydrogen** to form an **alkaline** solution.
- Down Group I the reactivity **increases**: lithium fizzes slowly; sodium melts to a silvery ball and its hydrogen may burn with a golden yellow flame; potassium fizzes furiously, with a **lilac** flame.
- **Alkaline earth metals ／ 鹼土金屬** (Group II) are less reactive than Group I, and reactivity **increases** down the group: beryllium does not react with water or steam, magnesium reacts readily with steam, calcium with cold water.
- **Halogens ／ 鹵素** (Group VII): fluorine (pale yellow gas), chlorine (greenish yellow gas), bromine (reddish brown liquid), iodine (black solid). They react with most metals to form **salts**.
- Down Group VII the colour becomes **darker** and the reactivity **decreases**: with hydrogen, fluorine reacts explosively even in the dark, but iodine does not react completely even on heating.
- **Uses of noble gases**: helium in balloons and airships (low density, unreactive), neon in advertising signs (glows red), argon in light bulbs (does not react with the filament).

## Must know

| Group | Name | Outermost shell electrons | Reactivity down the group | Key facts |
|---|-----|----|----|----------|
| **I** | alkali metals | 1 | **increases** | soft, low density; react with water → hydrogen + an alkaline solution; kept under paraffin oil |
| **II** | alkaline earth metals | 2 | **increases** | less reactive than Group I; react with dilute hydrochloric acid → hydrogen |
| **VII** | halogens | 7 | **decreases** | coloured, darker down the group; gas → liquid → solid; react with metals → salts (ionic compounds) |
| **0** | noble gases | 8 (helium: 2) | very unreactive | colourless gases; rarely react with other substances |

Groups I and II react with non-metals, and Group VII with metals, to form **ionic compounds**; Group VII reacts with non-metals to form **covalent compounds**.

## Common mistakes

- ✗ "All noble gases have 8 outermost shell electrons." ✓ **Helium** has 2 outermost shell electrons (a duplet structure); the others have 8.
- ✗ "Potassium is below sodium in Group I, so it is less reactive, just as bromine is less reactive than chlorine." ✓ Reactivity **increases** down Groups I and II but **decreases** down Group VII: potassium is more reactive than sodium.
- ✗ "Noble gases never react." ✓ They are very unreactive, not totally unreactive: xenon forms compounds such as XeF~2~.
- ✗ "Iodine is a purple solid." ✓ Solid iodine is **black**, iodine vapour is **purple** and aqueous iodine is **brown**.
- ✗ "Every Group I metal melts into a ball on water." ✓ Lithium floats and fizzes slowly but does **not** melt; sodium and potassium melt.

## Quick check

1. How many outermost shell electrons do atoms of (a) neon, (b) helium have? Which structure does each have?
2. Rubidium is below potassium in Group I. Compare their reactions with water.
3. Astatine is below iodine in Group VII. Predict its state, its colour and its reactivity compared with iodine.
4. Why must sodium be stored under paraffin oil?
5. Of beryllium, magnesium and calcium, which does not react with water or steam, and which reacts readily with cold water?

## Answers

1. Neon: 8, an octet structure. Helium: 2, a duplet structure.
2. Rubidium reacts even more vigorously than potassium: reactivity increases down Group I.
3. A black solid, less reactive than iodine: reactivity decreases down Group VII.
4. It is a reactive metal that reacts with air readily.
5. Beryllium does not react; calcium reacts readily with cold water.

## Did you know?

Helium was found in the Sun before it was found on Earth: in 1868 scientists saw an unknown yellow line in sunlight and named the element after "helios", the Greek word for the Sun.