Q77P - Chemistry: Molecular Nature Of Matter And Change

Question

The radii of the lithium and magnesium ions are 76 pm and 72 pm, respectively. Which compound has stronger ionic attractions, lithium oxide or magnesium oxide?

Step-by-Step Solution

Answer

Ionic attraction increases with increasing the charge and decreasing the size of the ions. Therefore, magnesium oxide has a stronger ionic attraction.

Step 1: Determine the trend of the size of the atom in the periodic table

On moving down the group the size of the atom increases due to the increase in the number of shells in the atom so, the attraction between the outermost electrons and the nucleus decreases and the size of the atom increases but on moving left to right in a period, electrons are added to the same shell, so the attraction between the nucleus and the outermost electrons increases, thus the size of the atom decreases when we move left to right in the period.

Step 2: Relation between the charge on the ion and ionic attraction

As the charge on cation or anion increases the attraction between the ions also increases as more charge will make the ion more attracted towards the opposite charge.

Since group 1A element loses one electron and group 2A element loses two electrons. Thus, Li loses one electron and forms ${\mathbf{Li}}^{\mathbf{+}}$ an ion, whereas magnesium loses two electrons and forms ${\mathbf{Mg}}^{\mathbf{+}\mathbf{2}}$ an ion. Therefore, the attraction between the magnesium ion $\left({\mathbf{Mg}}^{\mathbf{+}\mathbf{2}}\right)$ and the oxide ion $\left({\mathbf{O}}^{\mathbf{-}\mathbf{2}}\right)$ is stronger than the attraction between the lithium-ion $\left({\mathbf{Li}}^{\mathbf{+}}\right)$ and the oxide ion $\left({\mathbf{O}}^{\mathbf{-}\mathbf{2}}\right)$.

Hence, magnesium oxide has a stronger attraction than lithium oxide.