What are the various factors due to which the ionization enthalpy of the main group elements tends to decrease down a group?
The ionization enthalpy of the main group elements tends to decrease down a group primarily due to the increase in atomic and ionic radii with the increase in atomic number.
To understand this trend, two main factors are considered: (i) the attraction of electrons towards the nucleus, and (ii) the repulsion of electrons from each other. As we move down a group, the size of the atom increases, meaning the outermost electron is located farther from the positively charged nucleus.
Consequently, the attraction of electrons towards the nucleus decreases. At the same time, the addition of more inner electron shells increases the repulsion of electrons from each other. These combined effects make it easier to remove the outermost electron, resulting in a gradual decrease in ionization enthalpies down a group.
Explanation
The textbook context explains that ionization enthalpy and atomic radius are closely related properties. Down a group, atomic and ionic radii increase with atomic number. The two fundamental factors governing this trend are the attraction of electrons towards the nucleus and the repulsion of electrons from each other. As atomic size increases down a group, the outermost electrons are farther from the nucleus, reducing nuclear attraction and increasing electron-electron repulsion, which ultimately lowers the ionization enthalpy.