Electron shielding contributes to the increase in atomic size by diminishing the effective nuclear charge that the outermost electrons experience.

Electron shielding, also referred to as screening, is a phenomenon that occurs within atoms. It involves the ‘shielding’ or ‘screening’ effect of the inner shell electrons on the positive charge of the nucleus, which lessens the attractive force that the outer shell electrons feel. This reduction in attraction enables the outer electrons to occupy higher energy levels, thereby increasing the atomic size.

The concept of electron shielding is intrinsically linked to atomic structure. Atoms consist of a positively charged nucleus surrounded by negatively charged electrons organized into energy levels, or shells. The inner shell electrons, being closer to the nucleus, are strongly attracted to it due to the opposite charges. However, these inner shell electrons also exert a repulsive force on the outer shell electrons due to their like charges. This repulsion lowers the effective nuclear charge that the outer shell electrons experience, allowing them to drift further away from the nucleus.

The extent of electron shielding intensifies with the addition of more electron shells within an atom. More shells imply a greater number of inner shell electrons, which serve to shield the outer shell electrons from the nuclear charge. Consequently, atoms that possess more electron shells typically exhibit larger atomic sizes. For instance, potassium ($K$) has a larger atomic size compared to sodium ($Na$) because it contains one additional electron shell.

Moreover, electron shielding influences the trends in atomic size across the periodic table. As one moves from left to right across a period, the number of protons in the nucleus increases; however, electron shielding remains relatively constant since the electrons are added to the same energy level. This results in an increased effective nuclear charge, which pulls the outer electrons closer to the nucleus, consequently reducing the atomic size. Conversely, as one moves down a group, the number of electron shells increases, leading to enhanced electron shielding and a larger atomic size.

In summary, electron shielding is a vital factor in determining atomic size. It reduces the effective nuclear charge that outermost electrons experience, allowing them to drift further from the nucleus and thus increasing the overall atomic size.