The ionization energy (Ei) is qualitatively defined as the minimum amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated neutral gaseous atom to form a cation. It is quantitatively expressed in symbols as
X + energy →
X++e− where X is any atom or molecule capable of being ionized,
X+ is that atom or molecule with an electron removed, and
e− is the removed electron.
The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1). For example, the first three ionization energies are defined as follows:
1st ionization energy
X →
X++e− 2nd ionization energy
X+ →
X2++e− 3r ionization energy
X2+ →
X3++e− Generally, the (n+1)th ionization energy is larger than the nth ionization energy. When the next ionization energy involves removing an electron from the same electron shell, the increase in ionization energy is primarily due to the increased net charge of the ion from which the electron is being removed. Electrons removed from more highly charged ions of a particular element experience greater forces of electrostatic attraction;
thus, their removal requires more energy.
So for the given data,
KK+K2+ CaCa+Ca2+ d > c> b > a is the correct decreasing order of ionisation energy.
So, the option b > d is incorrect.