The number of lone pair of electrons on the central atom of XeF4 molecule is 

Lattice energy of ${\mathrm{BeCO}}_3\left(\mathrm{I}\right), {\mathrm{MgCO}}_3\left(\mathrm{II}\right)$ and ${\mathrm{CaCO}}_3\left(\mathrm{III}\right)$ is in order

Electronegativity values of elements help in predicting 

The pair of elements that would likely to form an ionic bond is