Compound Lattice Energy (kJ/mol) Compound Lattice Energy (kJ/mol) LiF 1030 NaCl 788 LiCl 834 MgCl2 2326 LiI 730 AlCl3 5376 Lattice Energy 1 Lattice Energy 2 For the lattice energies given above … in Lattice Energy 1, explain why LiF has a greater lattice energy than LiI in Lattice Energy 2, explain why NaCl has a smaller lattice energy than AlCl3

The lattice energy decreases down the group and increases across the period, therefore, the lattice energy of LiF > lattice energy of LiI, and the lattice energy of NaCl < the lattice energy of AlCl₃

Explanation:

The given parameters are;

Compound                                    Lattice Energy (kJ/mol)

LiF                                                       1030

NaCl                                                    788

MgCl₂                                                  2326

LiI                                                         730

AlCl₃                                                    5376

Lattice is the energy that must be added in order to split one mole of a ionic solid compound into gaseous ions

Given that the force of attraction between charges is inversely proportional to the square of the distance separating the charges, we have that as the atomic radius increases progressively down the group, the lattice energy decreases, therefore, LiF which has a shorter inter nuclear distance than LiI due to fluorine, F, being higher up on the periodic table than iodine, I, has a greater lattice energy than LiI

Similarly, given that the atomic radius decreases across the period, and that sodium, Na, comes before aluminum, Al, as we move across period 3, of the periodic table, the inter nuclear distance between atoms of in NaCl is larger than the internuclear distances between the atoms of AlCl₃, as well as the presence of more chlorine atoms in AlCl₃ than in NaCl, the lattice energy of NaCl is smaller than the lattice energy of AlCl₃.

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