The noble gases

The family that refuses to react

Sodium and chlorine both react fiercely, yet neon, the element between them in atomic number, will not react with anything ordinary. Why would one extra electron make an atom so calm?

Group 18 holds the noble gases: helium, neon, argon, krypton, xenon and radon. All are monatomic gases, existing as single atoms, and all have very low boiling points.

Each has eight electrons in its outer shell (s² p⁶), except helium, which has two (s²). Their outer shells are complete, so they have no need to gain, lose or share electrons and show very little chemical reactivity.

A complete outer shell means nothing to gain or lose: the noble gases are unreactive, monatomic gases.

Key terms

Noble gas

Naming the group.

Monatomic

Contrast with diatomic Cl₂.

Complete outer shell

Explaining low reactivity.

Boiling point

Why they are gases.

Short questions with model answers

1. Q1. X is a monatomic gas. Which statement is correct: X burns in air, X is coloured, X is unreactive, or X displaces iodine? (Exercise ix, p. 127)

   • Identify
   • Property

   X is unreactive.

2. Q2. Write the outer-shell arrangement of neon and of helium, and say why both are unreactive.

   • Ne: 2s² 2p⁶
   • He: 1s²

   Both outer shells are complete, so neither needs to gain, lose or share electrons.

Common mistakes

• ✗ Helium has eight outer electrons like the other noble gases.

  ✓ Helium has two (s²), which fills its only shell (p. 123).

• ✗ Noble gases exist as diatomic molecules like Cl₂.

  ✓ They are monatomic: single atoms (p. 123).

• ✗ Radon's symbol is Ra.

  ✓ Radon is Rn; Ra is radium. The book misprints this on p. 123.

MCQs

1. 1. X is a monatomic gas. Which statement is correct? (Exercise ix, p. 127)

   1. (a) X burns in air
   2. (b) X is coloured
   3. (c) X is unreactive
   4. (d) X displaces iodine
   Show answer

   (c) Complete outer shell (p. 123).

2. 2. Noble gases are in group:

   1. (a) 1
   2. (b) 2
   3. (c) 17
   4. (d) 18
   Show answer

   (d) p. 123.

3. 3. How many outer electrons does helium have?

   1. (a) 1
   2. (b) 2
   3. (c) 7
   4. (d) 8
   Show answer

   (b) s² (p. 123).

4. 4. Why do noble gases show very little reactivity?

   1. (a) They are metals
   2. (b) Their outer shells are complete
   3. (c) They are very heavy
   4. (d) They are liquids
   Show answer

   (b) Key point 9, p. 126.

5. 5. Which symbol is correct for radon?

   1. (a) Ra
   2. (b) Rn
   3. (c) Rd
   4. (d) R
   Show answer

   (b) Ra is radium; the book misprints radon as Ra.

Quick revision

• Group 18 noble gases are He, Ne, Ar, Kr, Xe and Rn: monatomic gases with very low boiling points.
• They have eight outer electrons (s² p⁶); helium has two (s²).
• Complete outer shells make them almost unreactive.