Chapter 7, Problem 75A

Interpretation Introduction

Interpretation:

The compounds from CsCl or KCl, K₂O or CaO which have highest lattice energy needs to be determined.

Concept introduction:

Molecules are bound together by different types of bonds namely ionic and covalent. Ionic bonds are formed when there is complete transfer of electrons from one atom to another. These atoms either lose or gain electrons to become negatively or positively charged ions. The forces of attraction between these ions causes the ionic bond formation and are strong bonds. Lattice energy is the energy released to break bonds.

Answer to Problem 75A

Of the compounds CsCl or KCl, K₂O or CaO, CaO will have highest lattice energy.

Explanation of Solution

Ionic bonds involve the transfer of electrons from one atom to the other. The metal atoms, an electrons donor, loses electrons to form a positively charged ion while the non-metal atoms, electron acceptor, gains electrons to form negatively charged ions. During the process of donating or gaining electrons, the atoms follow octet rule to attain a stable noble gas configuration.

ConsideringCsCl and KCl, K and Cs belongs to Group 1 and K fourth down the group while Cs is the sixth element down the group.

$\begin{array}{l}{\text{ K - 1s}}^{2}2{\text{s}}^{2}2{\text{p}}^{{}^6}3{\text{s}}^{2}3{\text{p}}^{{}^6}4{\text{s}}^1\\ {\text{Cs - 1s}}^{2}2{\text{s}}^{2}2{\text{p}}^{{}^6}3{\text{s}}^{2}3{\text{p}}^{{}^6}4{\text{s}}^{2}3{\text{d}}^{10}2{\text{p}}^{{}^6}4{\text{p}}^{{}^6}{\text{5s}}^{2}4{\text{d}}^{10}5{\text{p}}^{{}^6}6{\text{s}}^1\end{array}$

Cs will have many shells that causes less force of attraction between the nucleus and the outermost electrons. Due to less forces of attraction, the diameter of the ion formed will be large. But in case of K, the number of shells is less and hence diameter will be smaller than Cs.

When comparing the two compounds CsCl and KCl, the KCl will have highest lattice energy than CsCl.

Considering K₂O or CaO, K belongs to Group 1 and Ca belongs to group 2. But both belong to fourth period. This indicates that as we move along the period, the number of shells remains the same but the number of electrons increases.

$\begin{array}{l}{\text{ K - 1s}}^{2}2{\text{s}}^{2}2{\text{p}}^{{}^6}3{\text{s}}^{2}3{\text{p}}^{{}^6}4{\text{s}}^1\\ {\text{Ca - 1s}}^{2}2{\text{s}}^{2}2{\text{p}}^{{}^6}3{\text{s}}^{2}3{\text{p}}^{{}^6}4{\text{s}}^2\end{array}$

This causes more force of attraction between the nucleus and outermost electrons. Due to increased forces, the electrons are pulled closer to nucleus and hence their diameter decreases further. In other words, more energy is required to break the bonds or their lattice energy increases along period.

When comparing the compounds, K₂O or CaO., CaO will have higher lattice energy as it will have less diameter than K.

Making comparison between K and Ca, K will have +1 charge while Ca will have +2 charge. More is the positive charge then more is the lattice energy.

Conclusion

Of the compounds CsCl or KCl, K₂O or CaO, CaO will have highest lattice energy.