lattice energy

Lattice energy can be equivalently denoted as the attraction between the ions. i.e. F=kq_cationq_anion/(r_cat+r_an)²

In (1) the anion is same. So the lattice energy will depend on charge density of cation hence BeCO₃>MgCO₃>CaCO₃>SrCO₃>BaCO₃

In (2) similarly, Li₂CO₃>Na₂CO₃>K₂CO₃

In (3) LiCl>NaCl>KCl

In (4) cation is same so lattice energy depends on charge density of anion .. Hence LiF>LiBr>LiI

Q2.. As in both cases anion is same so the difference in lattice energy occurs due to the cations
Now Ca²⁺ is smaller in size than K⁺ and its charge is more.. Hence its charge density is more .So lattice energy of CaO is more than K₂O

q1.)(1) can be done using fajan's rule.
it tells, decrease in size of cations increases covalency.
the more covalent the moolecule, the lesser the lattice energy.
so, BeCO3<MgCO3<CaCO3<SrCO3<BaCO3

:O totally opposite answers...!