Group 17: the halogens

Two gases, a liquid and a solid in one family

Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid, yet all four are in group 17 and react in the same way. What changes as we go down?

Every halogen has seven outer electrons (ns² np⁵) and a strong pull to accept one more and become a negative ion. So they are electronegative, very reactive non-metals that exist as diatomic molecules such as Cl₂.

Down the group the atoms get larger with more electrons, so the molecules attract each other more strongly. Melting and boiling points rise, so bromine and iodine melt and boil higher than chlorine and fluorine. Fluorine is a pale yellow gas, chlorine yellowish-green, bromine a fuming red-brown liquid, and iodine shiny grey crystals that give purple vapour when warmed.

Halogens react with alkali and alkaline earth metals to give salts, which is why the name means 'salt-forming'. Unlike group 1, their reactivity decreases down the group: a bigger atom pulls in an extra electron less strongly.

Down group 17: bigger molecules, higher melting and boiling points, lower reactivity.

Key terms

Halogen

Naming the group.

Diatomic molecule

How halogens exist.

Electronegative

Explaining anion formation.

Metal halide

Product with alkali metals.

Short questions with model answers

1. Q1. Why does iodine exist as a solid at room temperature? (Short question iv, p. 127)

   • Size
   • Attraction

   Strong attraction between its large molecules gives iodine a high melting point, so it is solid.

2. Q2. Which halogen is the most reactive and which the least? Give the reason. (Constructed response vii, p. 128)

   • Trend
   • Reason

   Fluorine is the most reactive; iodine the least (of F, Cl, Br, I).

Common mistakes

• ✗ Halogens get more reactive down the group, like group 1.

  ✓ Opposite: halogen reactivity decreases down the group (p. 121).

• ✗ Chlorine gas is made of single Cl atoms.

  ✓ Halogens are diatomic: Cl₂ (p. 121).

• ✗ Melting points fall down group 17.

  ✓ They rise: bigger molecules attract more strongly (p. 121).

MCQs

1. 1. Which halogen will have the least reactivity with alkaline earth metals? (Exercise i, p. 126)

   1. (a) Chlorine
   2. (b) Iodine
   3. (c) Bromine
   4. (d) Fluorine
   Show answer

   (b) Lowest in the group of these four (p. 121).

2. 2. How many electrons are in a halogen atom's outer shell?

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

   (c) ns² np⁵ (p. 121).

3. 3. Bromine at room temperature is a:

   1. (a) Pale yellow gas
   2. (b) Red-brown liquid
   3. (c) Grey solid
   4. (d) Yellowish-green gas
   Show answer

   (b) p. 121.

4. 4. The word 'halogen' means:

   1. (a) Salt-forming
   2. (b) Colour-forming
   3. (c) Acid-forming
   4. (d) Gas-forming
   Show answer

   (a) They react with metals to give salts (p. 121).

5. 5. Why do melting points rise from F₂ to I₂?

   1. (a) Molecules get smaller
   2. (b) Larger molecules attract each other more strongly
   3. (c) Reactivity increases
   4. (d) They become ionic
   Show answer

   (b) p. 121.

Quick revision

• Halogens have seven outer electrons, form diatomic molecules and accept one electron to form anions.
• Down the group, melting and boiling points rise and reactivity falls.
• F₂ and Cl₂ are gases, Br₂ is a liquid and I₂ is a solid at room temperature.