When one mole of an ionic crystal is dissolved in water, water molecules gather from the ion and this process is called hydration. In this process, some amount of energy is released which is called Hydration energy.
Hydration energy of a metal cation increases with the increase in effective nuclear charge and decrease in ionic radii because these two factors bring the water molecules closer to the metal cation resulting in the increased electrostatic attraction between the metal cation and the water molecule.
For dipositive transition metal cation of 3d-series, the effective nuclear charge increases and ionic radii decrease across a period. So hydration energy should increase regularly from Ca2+ to Zn2+
So, Hydration energy α charge of the cation ̸ size of the cation
For example, Hydration energy of Co2+ < Co3+ because here as the size of both the ions are same, the ion having higher charge has greater hydration energy.
When one mole of an ionic crystal is formed from its constituent gaseous ions, some amount of energy is released which is called lattice energy.
Or energy required to break one mole of ionic crystal into its surrounding gaseous ion is called lattice energy.
According to Born Lande’s equation lattice energy of an ionic crystal increases with the increase in the product of Z+and Z–and decrease in the interionic distance(r0).
The lattice energy for the halides of dipositive metal ions of the 3d-series transition element should increase from Ca2+ to Zn2+ion and a straight line should be observed.
3.Ionic radii of Divalent Metal ions of 3d-series transition element:
The ionic radii of dipositive and tripositive metal cations of 3d-series transition metals in the low spin or high-spin octahedral field might be expected to decrease regularly from Ca2+to Zn2+ .
The reason is that there is an increase of force of attraction between metal cations and ligands due to the increase in effective nuclear charge and the poor shielding effect of d-electrons due to which ligands and metal cation approach each other more closely.