Enthalpy for the atomic / molecular change

i. Enthalpy of ionization ($\Delta_{ion}H$) : It is the enthalpy change accompanying the removal of an electron from one mole of gaseous atoms. For example,

The equation signifies that when one mole of gas-phase atomic sodium is ionized to gas-phase Na$^{+}$ ions, the enthalpy change is 494 kJ. The same amount of heat would be required to ionize one mole of Na atoms. (The book prints ionic charges in its circled style, Na$^{\oplus}$ and e$^{\ominus}$; we write the standard superscript charges with the same meaning.)

The electron gain enthalpy, on the other hand, gives the enthalpy change when one mole of gas-phase atoms of an element accept electrons to form gaseous anions. For example,

The electron gain enthalpy of Cl is -349 kJ mol$^{-1}$.

ii. Enthalpy of atomization ($\Delta_{atom}H$) : The enthalpy change accompanying the dissociation of one mole of gaseous substance into atoms is the enthalpy of atomization. Consider:

iii. Enthalpy of solution ($\Delta_{soln}H$) : Enthalpy of solution is the enthalpy change in a process when one mole of a substance is dissolved in a specified amount of solvent:

Enthalpy of solution at infinite dilution is the enthalpy change when one mole of a substance is dissolved in an infinite amount of solvent.

An ionic compound dissolves in water in two steps:

1. The ions are separated from the molecule:

The enthalpy change for this step is the crystal lattice enthalpy, $\Delta_L H$, which is always positive.

2. The ions are hydrated, with water molecules surrounding them:

The enthalpy change for this step is always negative, and is called the enthalpy of hydration, $\Delta_{hyd}H$. …