Question

Which of the following are the most ionic compounds?
A.${{\text{P}}_2}{{\text{O}}_5}$
B.${\text{Cr}}{{\text{O}}_3}$
C.${\text{MnO}}$
D.${\text{M}}{{\text{n}}_2}{{\text{O}}_7}$

Answer 1

We can calculate the percentage ionic and covalent character by using the Fajan’s rule. The greater is the charge on the cation the more covalent the species will be because anion is same in all the given options. The cation with least charge will be most ionic.

Complete step by step solution:
Ionic bond is a strong electrostatic force of attraction between cation and anion. Generally a cation is a metal and anion is a non metal. They are non directional in nature.
Fajan’s rule tells us about the percentage of covalent character in an ionic bond. It states that the cation with smaller size and more charge and anion with larger size and more charge results in more polarising power of cation. This all favours the formation of covalent character in ionic bond so we have to check charge in cation in each of the given options.
${{\text{P}}_2}{{\text{O}}_5}$ here the charge in phosphorous is 5
${\text{Cr}}{{\text{O}}_3}$ here the charge on chromium is 6
${\text{MnO}}$ here the charge on manganese is 2
${\text{M}}{{\text{n}}_2}{{\text{O}}_7}$ here charge on manganese is 7
Least charge is in ${\text{MnO}}$ and hence will be most ionic.

So, the correct option is C.

Note:
The favourable condition for the formation of ionic bond is low ionization energy of metal so that it will easily lose electrons and form a cation, high electron affinity of nonmetals so that it should accept the electron and forms an anion, large size cations are that have less polarizing power, small size Anion so that the polarization could be minimum, high lattice energy so as to minimise the energy of crystal for better stabilization and combining cation and anion must be different with electronegativity difference of at least $1.7$.