Question

Why is lithium carbonate insoluble in water? Does this not break the rule that the salts of all the alkali metals are soluble?

Answer 1

Lithium is the first element in the alkali metal series; however, it behaves differently from the other members. Lithium, unlike the other alkali metals, has a lower ionic character and forms covalent bonds. We also know that lithium has polarising properties and can deform the electron cloud of massive anions.

Complete Step By Step Answer:
The strength of the electrostatic attraction between the cations and anions that make up an ionic compound is directly proportional to their charge and inversely proportional to their distance apart.
This indicates that when charges are held constant, as they are for alkaline earth metal carbonates, the size of the alkaline earth metal ion is the most important factor affecting solubility.
Because the electrostatic force of attraction weakens with increasing cation size, alkaline earth metals with larger cations will be more soluble.
As the electrostatic interaction between the very small lithium cation and the carbonate anion overpowers the attraction between these ions and the water molecules, lithium carbonate is less soluble.
The compound will be soluble in water if the hydration energy of the ions is sufficient to overcome the lattice energy of the complex. It will not be soluble in water if the hydration energy of the ions does not exceed the lattice energy of the complex.

Note:
Aside from its strange behaviour, the lithium atom has a diagonal interaction with the alkaline earth element magnesium. In a period, an element's electropositivity increases down the group while decreasing from left to right. As a result of this combined effect, magnesium metal possesses an electro positivity similar to lithium.