Define ionization enthalpy. Discuss the factors affecting ionization enthalpy of the elements and its trends in the periodic table.

Ionization energy: it is the amount of energy required to remove an electron from an isolated gaseous atom and this is endothermic in nature.

Factors affecting ionization enthalpy are shielding effect, penetration effect, and electronic configuration.

Shielding effect: it is the repulsion(shielding) on the valence electrons by the core electrons. Due to this, the nuclear charge experienced by the electron reduces to an effective nuclear charge.

Z_eff = Z - σ

Where Zeff is the effective nuclear charge, Z is the nuclear charge, and σ is screening or shielding constant.

Penetration effect: it is the closeness of the electron present in the orbital to the nucleus. The orbital close to the nucleus will have a high penetrating effect so ionization energy will be high. The penetrating power of different orders in decreasing order-

2s > 2p > 3s > 3p > 4s > 3d

Electronic configuration: the orbitals that are half-filled or full-filled, the ionization enthalpy is higher as they are more stable.

It is the energy to remove an electron and it is endothermic(energy given from outside) in nature