Chapter 7, Problem 10STP

Interpretation Introduction

Interpretation: The trend in lattice energy needs to be explained if the atomic radius of halogen increases.

Concept Introduction: Lattice energy is defined as the amount of energy needed to form gaseous ionic components of a 1 mol ionic solid. It is the energy required to separate ionic crystals to form their gaseous ions. Since energy is needed, the process is endothermic in nature.

Answer to Problem 10STP

The lattice energy decreases if the atomic radius of halogen increases.

Explanation of Solution

From the graph, it can be seen that lattice energies for ionic compounds formed between alkali metals and fluorine have maximum values when compared with ionic compounds formed by other halogens. Also, ionic compounds formed between alkali metals and iodine have minimum values.

It can be seen that the lattice energy decreases while moving from top to bottom in a group in the case of halogens.

Therefore, the increasing order of atomic radius will be:

  $\text{I}>\text{Br}>\text{Cl}>\text{F}$

Now, atomic radius increases from top to bottom thus, if atomic radius increases, lattice energy decreases.

Conclusion

Thus, as the atomic radius of halogen increases, the lattice energy of the ionic compounds formed between alkali metals and halogens decreases.