Halides: from table salt to a fuming liquid in one row

The problem

Sodium chloride is the salt on your dinner table: a hard crystal that melts only at about 801 °C. Walk right along period 3 and silicon tetrachloride is already a liquid at room temperature, freezing only at −68 °C. Both are simply “element + chlorine”.

The difference is the bond. On the left, chlorine takes an electron outright and forms ions. Further right, it only shares. How electropositive the other element is decides which happens.

Halides are binary compounds of the halogens with other elements. Their physical properties are largely decided by their bonding: ionic, polymeric or covalent.

Key terms

Ionic halides

Strongly electropositive elements, with a large electronegativity difference from the halogen, form ionic halides. Group 1 (IA) halides are purely ionic: high-melting solids with three-dimensional lattices of discrete ions.

Polymeric halides

Here a halogen atom acts as a bridge between two atoms of the other element. Less electropositive elements such as Be, Ga and Al form them, with partly ionic bonding and layer or chain lattices.

Covalent halides

Towards the right of a period the electronegativity difference shrinks and the halides become covalent molecules. Only weak van der Waals forces hold them together, so they are gases, liquids or low-melting solids.

Oxidation state rule

When an element forms more than one halide, the halide in the lower oxidation state tends to be ionic and the one in the higher state covalent. PbCl₂ is mainly ionic; PbCl₄ is fairly covalent, because Pb⁴⁺ has a high polarizing power.

Short questions with model answers

1. Q1. Give a brief reason: the ionic character of halides decreases from left to right in a period.

   • Left to right, elements become less electropositive.
   • The electronegativity difference shrinks, so electrons are shared, not transferred.
   • NaCl 801 → SiCl₄ −68 °C

   The electronegativity difference with the halogen decreases, so the bonding shifts from ionic to covalent.

2. Q2. AlF₃ melts at 1290 °C and conducts; AlI₃ melts at 198 °C (as printed) and does not. Explain.

   • Ionic character falls down the halogens: fluoride > chloride > bromide > iodide.
   • 1290 − 198 = 1092 °C

   AlF₃ is purely ionic; AlI₃ is predominantly covalent, with no free ions to carry current.

3. Q3. Lead forms PbCl₂ and PbCl₄. Which is more covalent, and why?

   • PbCl₂: Pb²⁺ · PbCl₄: Pb⁴⁺
   • Pb⁴⁺ has a much higher polarizing power.

   PbCl₄: the higher oxidation state gives the covalent halide.

Common mistakes

• ✗ Using one order for all halides: “iodides always melt highest”.

  ✓ Ionic halides: fluorides highest (small F⁻, highest lattice energy). Covalent halides: iodides highest (most polarizable, strongest van der Waals forces).

• ✗ Assuming a higher charge on the metal makes its halide more ionic.

  ✓ The opposite: the higher oxidation state tends to be covalent, as with PbCl₄, because the small, highly charged cation polarizes the halide.

• ✗ Being thrown by NaCl's melting point: the book prints 808 °C.

  ✓ The standard value is 801 °C. Either way the trend is the same: NaCl has the highest melting point of the period 3 chlorides.

MCQs

1. 1. The halides of group 1 (IA) are:

   1. (a) purely covalent
   2. (b) polymeric
   3. (c) purely ionic
   4. (d) gases
   Show answer

   (c) Purely ionic: high-melting solids with three-dimensional lattices of discrete ions.

2. 2. Halides in which the halogen acts as a bridge between two atoms of the other element are called:

   1. (a) ionic
   2. (b) polymeric
   3. (c) covalent
   4. (d) hydrated
   Show answer

   (b) Polymeric halides, formed by less electropositive elements such as Be, Ga and Al.

3. 3. Which chloride in Table 1.3 has the lowest melting point?

   1. (a) SiCl₄
   2. (b) PCl₃
   3. (c) S₂Cl₂
   4. (d) AlCl₃
   Show answer

   (b) PCl₃ at −93 °C, just below S₂Cl₂ (−80 °C) and SiCl₄ (−68 °C).

4. 4. Which lead chloride is fairly covalent?

   1. (a) PbCl₂
   2. (b) PbCl₄
   3. (c) both
   4. (d) neither
   Show answer

   (b) PbCl₄: lead in its higher oxidation state (+4) has a high polarizing power.

5. 5. For a metal, the order of decreasing ionic character of its halides is:

   1. (a) iodide > bromide > chloride > fluoride
   2. (b) fluoride > chloride > bromide > iodide
   3. (c) chloride > fluoride > iodide > bromide
   4. (d) all equal
   Show answer

   (b) Fluoride > chloride > bromide > iodide: the electronegativity difference shrinks down the halogens.

Quick revision

• Halides are ionic, polymeric or covalent; the electronegativity difference with the halogen decides which.
• Across period 3 the chlorides go from ionic NaCl (801 °C) to covalent SiCl₄, PCl₃ and S₂Cl₂, below 0 °C.
• Down the halogens ionic character falls (AlF₃ ionic, AlI₃ covalent); a higher oxidation state gives a more covalent halide (PbCl₄).