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In this lesson, we will learn:
- To understand why theoretical and experimental values for lattice energy are different.
- To explain how polarization leads to ionic bonds with covalent character.
- To understand the factors that affect the polarizability of anions in an ionic compound.
- When we calculate lattice energy for a compound, there is an issue to deal with. We can get two different values depending on how we find it.
- An experimental value that comes from Born-Haber cycles, calculated using enthalpy data (e.g. △Hf , ionization energy, atomization enthalpy) taken from experiments.
- A theoretical value which comes from electrostatic theory that explains how charges interact (Coulombic attraction).
- When we describe ionic bonding, the ions are often shown:
- Shaped like perfect spheres (or point charges) and
- Not making contact with each other.
- Remember, the cation polarises and the anion gets polarised, not the other way around!
- Polarising power: This is the ability of the cation to distort the anion’s electron cloud. This makes the bond more covalent in character and two factors make this effect stronger:
- Charge: the larger the charge, the more polarising the cation will be to any nearby negative ions.
- Ionic radius: the smaller the ionic radius, the higher the charge density which makes the cation’s polarising effect stronger.
- Polarisability: This is how easily the anion has its electron cloud distorted. More distortion creates a bond with more covalent character. Polarisability is affected by:
- Ionic radius: the larger the ionic radius, the greater the polarisability of an ion because electrons are held further from the positive nucleus they’re attracted to.
- This covalent character is seen in the difference between theoretical and experimental values for lattice enthalpy. The theory assumes perfect ionic bonding – the experimental reality, with covalent character, increases the lattice enthalpy. The gap between the two values follows a pattern with the gap in electronegativity between the two given ions in a compound - a gap in electronegativity is what drives ionic bonding in the first place!
If these values are different, then the theory does not describe the real ‘ionic’ lattice with 100% accuracy. This is because the bonding in the lattice is not 100% ionic!
There are two properties to know: