*"Electron gain enthalpy is energy released when one mole of electrons is added to gaseous atoms of an element".

*Positive electron gain enthalpy means energy needs to be supplied so that an electron can enter the atom.
Mg are electropositive metallic elements, which become unstable on adding electrons.

*∴ Electron gain enthalpy of   Mg care positive.

Introduction:
The ionization enthalpy of an element is the energy required to remove an electron from a gaseous atom or ion. In the case of magnesium (Mg), the ionization enthalpy is positive, indicating that energy is needed to remove an electron from a magnesium atom. This can be explained by considering the electronic configuration and atomic structure of magnesium.

Electronic Configuration:
Magnesium has an atomic number of 12, which means it has 12 electrons. The electronic configuration of magnesium is 1s² 2s² 2p⁶ 3s². The outermost shell, known as the valence shell, contains only two electrons in the 3s orbital.

Shielding Effect:
The positive charge of the nucleus in magnesium is balanced by the negatively charged electrons. However, the inner electrons shield the outermost electrons from the full force of the positive charge. This shielding effect reduces the attractive force between the nucleus and the valence electrons, making it easier to remove an electron.

Effective Nuclear Charge:
The effective nuclear charge experienced by the valence electrons is less than the actual nuclear charge due to the shielding effect. This reduced effective nuclear charge makes it easier to remove an electron from the valence shell.

Distance from the Nucleus:
The valence electrons in magnesium are located in the third energy level, which is relatively far from the nucleus compared to the inner electrons. This increased distance weakens the attraction between the nucleus and the valence electrons, making it easier to remove an electron.

Conclusion:
The positive ionization enthalpy of magnesium can be explained by its electronic configuration, shielding effect, and the distance of the valence electrons from the nucleus. The presence of inner electrons reduces the effective nuclear charge experienced by the valence electrons, while their distance from the nucleus weakens the attractive force. As a result, energy is required to remove an electron from a magnesium atom, leading to a positive ionization enthalpy.