How lattice enthalpies of alkaline earth metal carbonates is affected on moving down the group?
Lattice enthalpies are defined as the change in energy when one mole of crystal lattice is turned into infinite separated gaseous ions. When salts are dissolved in water, we assume the above definition of lattice enthalpy. Lattice enthalpies are affected by two main factors:
- Charge (larger charge = stronger enthalpy)
- Size of ion (smaller ion = stronger enthalpy)
Now, because you are trying to bond M (alkali earth metals/group 2) to carbonates, then the charge is going to stay constant. This is because carbonates (CO3) will always have a charge of -2 and M will always have a charge of +2. So, we are going to forget about charges for your specific situation.
If you look downwards on a periodic table in G2, the ionic radius of alkali earth metals increases. The smallest, and thus the one that will result in the strongest lattice enthalpy, is Beryllium. The smaller ionic radii allow for tighter packing, which leads to greater lattice enthalpy values.