Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.15 Polarisation and covalency-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.15 Polarisation and covalency- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 12.15 Polarisation and covalency- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.

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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics

12.15 Polarisation and Covalent Character in Ionic Bonds

Although ionic bonding is often described as purely electrostatic attraction between oppositely charged ions, in reality many ionic compounds show some degree of covalent character. This arises due to polarisation, where the electron cloud of an anion is distorted by a cation. Evidence for this comes from comparisons of lattice energies using Born–Haber cycles.

Polarisation

Polarisation is the distortion of the electron cloud of an anion by a cation.

How Polarisation Leads to Covalent Character

  • A highly charged or small cation attracts electrons strongly.
  • This pulls electron density away from the anion.
  • The electron cloud becomes distorted rather than spherical.
  • Electron density is partially shared between the ions.
  • This introduces covalent character into the ionic bond.

Evidence from Born–Haber Cycles

  • Experimental lattice energy is determined using a Born–Haber cycle.
  • Theoretical lattice energy assumes purely ionic bonding.
  • When polarisation occurs, additional attraction due to covalent character is present.
  • This makes the experimental lattice energy more exothermic than the theoretical value.

Factors Affecting Polarisation (Fajans’ Rules)

Cation:

    • Small ionic radius → higher charge density → greater polarising power.
    • Higher charge → stronger attraction of electrons.

Anion:

    • Large ionic radius → electron cloud easily distorted.
    • Higher negative charge → greater repulsion between electrons → easier distortion.

Consequences of Polarisation

  • Increased covalent character in the bond.
  • Greater deviation between experimental and theoretical lattice energies.
  • Physical properties may differ from purely ionic compounds (e.g. lower melting point).

Key Features

  • Polarisation causes partial electron sharing.
  • Leads to covalent character in ionic compounds.
  • Evidenced by more exothermic experimental lattice energy.
  • Depends on size and charge of ions.

Example 1:

Explain why \( \mathrm{AlCl_3} \) shows significant covalent character.

▶️ Answer/Explanation

The \( \mathrm{Al^{3+}} \) ion is small and highly charged, giving it a very high charge density.

This strongly polarises the electron cloud of the \( \mathrm{Cl^-} \) ion.

The electron cloud becomes distorted and electron density is shared.

Therefore, significant covalent character is present in \( \mathrm{AlCl_3} \).

Example 2:

Explain why \( \mathrm{MgO} \) has less covalent character compared to \( \mathrm{AlCl_3} \).

▶️ Answer/Explanation

Although \( \mathrm{Mg^{2+}} \) has a positive charge, it has a lower charge density than \( \mathrm{Al^{3+}} \).

The \( \mathrm{O^{2-}} \) ion is smaller than \( \mathrm{Cl^-} \) and less easily polarised.

Therefore, there is less distortion of the electron cloud.

As a result, \( \mathrm{MgO} \) remains more ionic with less covalent character.

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