Factors affecting lattice enthalpy

2 factors that affect the lattice enthalpy ?

The size and charge of the ions

The size of ions

hailde ion increases in size, as going down the group because of an increase of electrons, distance between the+ and - ions increases

Electrostatic force of attraction between the ions decreases leading to → lattice enthalpy value decrease.

The charge of ions

The greater the charters on the ion, the stronger the electrostatic force of attraction between the ions.

Therefore more energy is required to separate the ions and lattice enthalpy value increases.

Atomic and Ionic radii 

Cation is it smaller or bigger than atoms from which it formed ? Why 

Cation is smaller than atoms from which it formed, because it lost an electron leading to an increase of electrostatic force of attraction. 

Onion is it smaller or bigger than atoms from which it formed ?

Onion is bigger than atoms from which it formed because it gained  an electron leading to an decrease of electrostatic force of attraction. 

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what does an perfect ionic model look like ?

Perfect sphere, held by electrostatic force only

Alkali Halide

is an alkali halide a perfect ionic model ?

Yes, because of the agreement between the lattice enthalpy (dissociation ) from the Born Haber cycle and perfect model

what does an ionic bonding look like - prefect ionic model 

Does not take into account any covalent bonding present between the ions. 

what does an ionic / covalent bond look like - not perfect ionic model 

Not perfect sphere + some covalent bonding between the ions 

Type of bond : Silver halides is ionic with covalent character.

why does it require large amount of energy to convert ionic ions to aq ions ?

As the bonding is stronger, higher value of lattice enthalpy from the born hater cycle compared to prefect ionic model.

Positive and negative ions

electron cloud of the - negative ions can be (distorted) polarised by the + positive ions.

Negative ions are easily polarised if they are large

iodide ion -

Large and easily distorted / polarised → the ionic bond has the greatest covalent character.

Covalent character 

When a compound has some covalent character - it tends to wards giant covalent so the lattice is stronger than it was 100 % ionic. 

therefore born haber value would be larger than theoretical one.