Explanation:

When an atom loses one or more electrons to form a cation, it becomes positively charged. This loss of electrons creates a change in the electron configuration and affects the size of the ion. The cationic radius is always smaller than the atomic radius of the parent atom, and this can be explained by the following factors:

1. Loss of electron(s):
When an atom loses one or more electrons to form a cation, the number of electrons decreases. The loss of negatively charged electrons reduces the electron-electron repulsion within the electron cloud, allowing the remaining electrons to be pulled closer to the nucleus. This results in a decrease in the ionic radius compared to the atomic radius.

2. Effective nuclear charge:
The effective nuclear charge experienced by the remaining electrons in a cation increases. The effective nuclear charge is the net positive charge experienced by an electron, taking into account the shielding effect of other electrons. With the loss of electrons, the shielding effect decreases, and the remaining electrons are more strongly attracted to the nucleus. This increased attraction results in a contraction of the electron cloud and a decrease in the ionic radius.

3. Electron-electron repulsion:
The loss of electrons also reduces the electron-electron repulsion within the electron cloud. In an atom, electrons repel each other due to their negative charges. When electrons are lost to form a cation, this repulsion is reduced, allowing the remaining electrons to come closer together. This contraction of the electron cloud further contributes to a smaller ionic radius compared to the atomic radius.

Conclusion:
In summary, the cationic radius is always smaller than the atomic radius because of the loss of electrons, increased effective nuclear charge, and reduced electron-electron repulsion. This decrease in size can be observed across the periodic table for cations formed from different elements.