Noble gases

O Level / IGCSE Chemistry · The Periodic Table. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The noble gases are the elements of Group VIII: helium, neon, argon, krypton and xenon. They are all colourless gases at room temperature.

Each noble gas atom has a full outer shell of electrons. Helium has 2 electrons, which fills the first shell; the others have 8 in their outer shell. A full outer shell is a stable arrangement, so the atoms do not lose, gain or share electrons. This is why the noble gases are unreactive.

Because the atoms do not bond, even with each other, the gases are monatomic: they exist as single atoms such as He and Ar, not as molecules like O2 or Cl2.

Rules to remember

Common mistake

Writing that every noble gas has 8 outer electrons: helium has only 2, but its outer shell is still full. Also, they are not unreactive because they are gases; the reason is the full outer shell.

Worked example

Element Z has proton number 10. Will Z react with sodium, and what particles does a sample of Z contain?

  1. 10 protons means 10 electrons, arranged 2,8.
  2. The outer shell holds 8 electrons, so it is full and stable.
  3. A Z atom does not gain, lose or share electrons, so it does not react with sodium or bond to other Z atoms.

Answer: Z (neon) does not react with sodium and exists as single atoms: it is a monatomic gas.

Practice questions

Try each one, then open the answer.

1. Why do noble gases exist as single atoms rather than as molecules?

  1. A
    their atoms are too heavy to form molecules
  2. B
    their atoms have no electrons
  3. C
    they are made of ions
  4. D
    their atoms have full outer shells and do not form bonds
Show answer

Answer: D. A full outer shell is a stable arrangement, so noble gas atoms do not share, gain or lose electrons. They are monatomic: He, Ne, Ar.

2. Neon (2,8) forms no compounds, but fluorine (2,7) is very reactive. Why?

  1. A
    neon has a full outer shell, so it does not gain, lose or share electrons
  2. B
    neon has more protons than fluorine
  3. C
    neon is a liquid at room temperature, so its atoms cannot collide
  4. D
    fluorine has a full outer shell, so it takes part in more reactions
Show answer

Answer: A. Fluorine needs one more electron to fill its outer shell, so it reacts readily. Neon’s outer shell is already full.

3. Which statement about the noble gases is correct?

  1. A
    they are metals with very high melting points
  2. B
    they exist as diatomic molecules
  3. C
    they form 1− ions
  4. D
    they are unreactive gases made of single atoms
Show answer

Answer: D. Group VIII elements are monatomic gases with full outer shells, so they are unreactive.

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