Ionic compounds generally consist of positively charged ions (cations) and negatively charged ions (anions) held together by strong electrostatic attractions. Their formation and stability depend on the overall energy changes involved in converting the elements into gaseous ions and then forming the ionic crystal.
1. Low Ionisation Enthalpy
Ionisation enthalpy is the enthalpy required to remove an electron from an isolated gaseous atom to form a gaseous cation.
- Requirement: A metal forming a cation should generally have low ionisation enthalpy.
- Effect: Lower ionisation enthalpy makes removal of an electron energetically easier.
- Example: Alkali metals such as sodium and potassium have relatively low first ionisation enthalpies and readily form M+ ions.
2. Favourable Electron Gain Enthalpy
Electron gain enthalpy is the enthalpy change associated with the addition of an electron to an isolated gaseous atom to form a gaseous anion.
- Requirement: Formation of an anion is generally favoured when the atom has a more negative electron gain enthalpy.
- Effect: A more favourable electron gain enthalpy makes electron addition energetically more favourable.
- Example: Halogens generally have favourable electron gain enthalpies and readily form X− ions.
3. High Lattice Enthalpy of Formation
Lattice enthalpy can be defined, using the formation convention, as the enthalpy change when one mole of an ionic solid is formed from its constituent gaseous ions.
- Requirement: Formation of an ionic solid is strongly stabilised by a large magnitude of lattice enthalpy.
- Effect: Strong electrostatic attraction between the ions provides substantial lattice stabilisation.
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Factors favouring a large lattice enthalpy magnitude:
- Small ionic radii: Smaller ions can approach each other more closely.
- High ionic charges: Greater ionic charges produce stronger electrostatic attraction.
According to Coulombic considerations, electrostatic attraction increases with ionic charge and decreases as the distance between the ion centres increases.
4. Electronegativity Difference
Electronegativity difference provides an indication of the polarity and ionic character of a bond between two atoms.
- General trend: A larger electronegativity difference generally corresponds to greater ionic character.
- Important: The often-quoted value of about 1.7 on the Pauling scale is only an approximate guideline, not a strict boundary separating ionic and covalent bonds.
- Example: The bond in sodium chloride has substantial ionic character because of the large difference in electronegativity between sodium and chlorine.
5. Overall Energy Change
The formation of an ionic compound cannot be predicted reliably from only one factor. The overall thermodynamic energy balance is important.
- Ionisation enthalpy is required to form the gaseous cation.
- Electron gain enthalpy contributes to formation of the gaseous anion.
- Lattice enthalpy provides substantial stabilisation when the gaseous ions form the ionic solid.
- In a Born–Haber cycle, these and other relevant enthalpy changes are considered together to analyse the formation of an ionic solid.
Conclusion
The formation and stability of ionic compounds are influenced by the ease of cation formation, the favourability of anion formation, the magnitude of lattice stabilisation and the electronegativity difference between the combining atoms. Ultimately, the overall thermodynamic energy balance determines whether formation of the ionic compound is favourable.
Must Read Characteristics of Ionic Compounds
Asked in B.Sc. 2nd Semester 2025 (LOYOLA COLLEGE (AUTONOMOUS) CHENNAI)