An ionic compound $A^{+}{B}^{-}$ is formed by the transfer of electrons from atom A to atom B. For this process to be energetically favorable, certain conditions regarding the ionization enthalpy of A and electron affinity of B must be met.

Concept:

Formation of an ionic bond involves the complete transfer of one or more electrons from a metal atom (which forms a cation) to a non-metal atom (which forms an anion). For this to occur readily:

• The metal atom (A) should have a low ionization enthalpy, meaning it requires less energy to remove an electron and form a positive ion ($A^{+}$).
• The non-metal atom (B) should have a high electron affinity, meaning it releases a significant amount of energy when it accepts an electron to form a negative ion ($B^{-}$).

These conditions ensure that the overall energy change for the formation of the ionic compound is favorable, leading to a stable ionic bond.

Why (B) is correct:

If the ionization enthalpy of A is low, A can easily lose an electron to form $A^{+}$. If the electron affinity of B is high, B can easily accept an electron to form $B^{-}$. These two conditions together facilitate the electron transfer and the formation of a stable ionic compound $A^{+}{B}^{-}$.

Option Analysis:

• A) the ionization enthalpy of A high and electron affinity of b is low. This would make it difficult for A to lose an electron and for B to accept an electron, thus hindering ionic bond formation.
• B) the ionization enthalpy of A is low and electron affinity of B is high. This is the ideal condition for the formation of an ionic compound, as A readily loses an electron and B readily accepts it.
• C) both, the ionization enthalpy of A and electron affinity of B are high. High ionization enthalpy of A makes electron removal difficult, and while high electron affinity of B is good, the difficulty in forming $A^{+}$ would impede the process.
• D) both, the ionization enthalpy of A and electron affinity of B are low. Low ionization enthalpy of A is good, but low electron affinity of B means B does not readily accept an electron, making ionic bond formation less favorable.

Correct Answer: (B)