3.1.4 Covalent Character in a Compound

This topic explains how ionic compounds can have some covalent character, and what factors determine the degree of covalent character in a compound.

Definition

Covalent Character: The partial sharing of electrons between ions in an ionic compound, giving the bond some covalent nature.

Polarising Power: The ability of a cation to distort the electron cloud of an anion.

Main Idea

Covalent character in a molecule depends upon the polarising power of the cation, which in turn depends on the oxidation state of the cation. Higher the polarising power of the cation, more will be the covalent character. That is why when we move from left to right in periods, covalent character increases.

Core Concept

Even in ionic compounds, the cation can attract the electron cloud of the anion, causing some electron sharing. This is called polarisation. The more the anion is polarised, the more covalent character the compound has.

Detailed Explanation

What is Covalent Character?

• In an ideal ionic bond, electrons are completely transferred from the cation to the anion.
• In reality, the cation can distort or polarise the electron cloud of the anion.
• This distortion causes some sharing of electrons between the two ions.
• The result is that the bond has some covalent character.
• The more the anion is polarised, the more covalent character the compound has.

Factors Affecting Covalent Character

Covalent character depends upon the polarising power of the cation.

• Oxidation state (charge) of the cation: Higher charge → higher polarising power → more covalent character.
• Size of the cation: Smaller cation → higher polarising power → more covalent character.
• Size of the anion: Larger anion → more easily polarised → more covalent character.
• Electronic configuration of the cation: Cations with non-noble gas configuration have higher polarising power.

Example: Aluminium Chloride vs. Magnesium Chloride

• Oxidation state of aluminium = +3.
• Al³⁺ has a high charge and small size.
• High polarising power → can polarise the anion strongly.
• Electrons are shared between the two ions.
• Covalent character is high.

• Oxidation state of magnesium = +2.
• Mg²⁺ has a lower charge and larger size than Al³⁺.
• Lower polarising power → weaker polarisation of the anion.
• Covalent character is lower.

• Aluminium chloride is more covalent in nature as compared to magnesium chloride.
• This is because of the high polarising power of Al³⁺, which can polarise the anion, causing electrons to be shared between the two ions.

Trend Across a Period

• When we move from left to right in periods, the oxidation state of the cation increases.
• Higher oxidation state → higher polarising power → more covalent character.
• That is why covalent character increases from left to right across a period.

• NaCl (Na⁺, +1) → less covalent
• MgCl₂ (Mg²⁺, +2) → more covalent than NaCl
• AlCl₃ (Al³⁺, +3) → more covalent than MgCl₂

Fajan's Rules

• High charge on the cation
• Small size of the cation
• Large size of the anion
• Cation with non-noble gas configuration (e.g., transition metal ions)

Intermediate Level

Polarising Power

The ability of a cation to distort the electron cloud of an anion is called its polarising power.

• Charge on cation: Higher charge → higher polarising power.
• Size of cation: Smaller size → higher polarising power.
• Size of anion: Larger anion → more easily polarised.
• Electronic configuration: Non-noble gas configuration → higher polarising power.

Covalent Character

• Higher polarising power → more covalent character.
• Lower polarising power → more ionic character.

Example: AlCl₃ vs. MgCl₂

Conclusion: AlCl₃ is more covalent than MgCl₂.

Trend Across a Period

• Oxidation state increases from left to right.
• Polarising power increases.
• Covalent character increases.

Important Points

• 1. Covalent character depends on the polarising power of the cation.
• 2. Higher polarising power → more covalent character.
• 3. Polarising power depends on charge, size, and electronic configuration.
• 4. AlCl₃ is more covalent than MgCl₂.
• 5. Covalent character increases from left to right across a period.
• 6. Fajan's rules describe factors affecting covalent character.

Remember

• - Covalent character: Partial sharing of electrons
• - Polarising power: Ability of cation to distort anion
• - Factors: Charge, size, electronic configuration
• - Example: AlCl₃ > MgCl₂ (covalent character)
• - Trend: Increases from left to right across a period

Common Mistakes

• 1. Thinking ionic compounds have no covalent character: Even ionic compounds have some covalent character due to polarisation.
• 2. Forgetting the role of cation size: Smaller cation → higher polarising power.
• 3. Ignoring the role of anion size: Larger anion → more easily polarised.
• 4. Confusing polarising power with electronegativity: Polarising power is about distorting an anion's electron cloud; electronegativity is about attracting shared electrons.

Quick Revision

• Covalent character depends on the polarising power of the cation.
• Higher polarising power → more covalent character.
• Polarising power depends on charge, size, and electronic configuration.
• AlCl₃ is more covalent than MgCl₂.
• Covalent character increases from left to right across a period.
• Fajan's rules describe factors affecting covalent character.

60-Second Revision

• Covalent character: Partial sharing of electrons
• Polarising power: Ability of cation to distort anion
• Factors: Charge, size, electronic configuration
• Example: AlCl₃ > MgCl₂ (covalent character)
• Trend: Increases from left to right across a period

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