Question

Does lattice energy increase across a period?

Answer 1

We need to know that the lattice energy is the energy required to break an ionic solid into gaseous ions. Lattice energy is a measure of the strength of the ionic bonds in an ionic compound. It provides insight into several properties of ionic solids including their volatility, solubility and hardness. The lattice energy of an ionic solid cannot be measured directly.

Complete answer:
As we know that the lattice energy increases as we move across a period. Generally, there are two main requirements for increasing lattice energy. These include:
By decreasing the atomic radius: So when we move across a period, the atomic radius of elements decreases wherein protons are pulling the electrons more and more. Thus, as the radius decreases, lattice energy increases.
Secondly, when there is an increase in an ion’s charge. When we move across a period, the charge in the ions increases such as ${K^ + },{\text{ }}C{a^{2 + }},{\text{ }}G{a^3}$+ etc. As a result of this, the lattice energy also increases.
The formation of a crystal lattice is exothermic, i.e., the value of $\Delta {H_{lattice}}$ is negative because it corresponds to the coalescing of infinitely separated gaseous ions in vacuum to form the ionic lattice. In the case of $NaCl$, lattice energy is the energy released by the reaction.
$N{a^ + }\left( g \right) + C{l^ - }\left( g \right) \to NaCl\left( s \right)$
Which would amount to $- 786kJ/mol$.

Note:
We have to know that the concept of lattice energy was originally developed for rock salt-structured and sphalerite-structured compounds like $NaCl$ and $ZnS$, where the ions occupy high-symmetry crystal lattice sites. The bond between ions of opposite charge is strongest when the ions are small.