P^3$$-$$ > S^2$$-$$ > Cl^$$-$$ > K⁺ > Ca²⁺ (Correct order of ionic radii)

all the given species are isoelectronic species. In isoelectronic species size increases with increase of negative charge and size decreases with increase in positive charge.

a. Standard enthalpy of formation for alkali metal bromides becomes more negative on descending down the group.

b. In case of CsI, lattice energy is less, but Cs⁺ is having less hydration enthalpy due to which it is less soluble in water.

c. For alkali metal fluorides, the solubility in water increases from lithium to caesium. LiF is least soluble in water.

d. Standard enthalpy of formation for LiF is most negative among alkali metal fluorides.

Type of back bonding

$$\matrix{ {B{F_3}} & {BCl} & {BB{r_3}} & {B{I_3}} \cr {(2p\pi - 2p\pi )} & {(2p\pi - 3p\pi )} & {(2p\pi - 4p\pi )} & {(2p\pi - 5p\pi )} \cr } $$

Therefore back bonding strength is as follows

BF₃ > BCl > BBr₃ > BI₃

Group 16/oxygen family is known as Chalcogens the members are O, S, Se, Te, Po