What is electron shielding and its influence on periodic properties?

Electron shielding is the decrease in attraction between an electron and the nucleus due to intervening electron layers.

Electron shielding, also known as screening, is a key concept in atomic structure and periodic properties. It refers to the phenomenon where core electrons, those in inner energy levels, shield or screen the outer (valence) electrons from the full positive charge of the nucleus. This is because the negatively charged electrons in the inner shells repel the electrons in the outer shells, reducing the effective nuclear charge experienced by the outer electrons.

The degree of electron shielding can greatly influence an atom's size, ionisation energy, electron affinity, and electronegativity. For instance, as you move across a period from left to right, the number of protons in the nucleus increases, as does the number of electrons. However, these additional electrons are added to the same energy level, not intervening ones, so the shielding effect remains constant. The increasing nuclear charge pulls the electrons closer to the nucleus, decreasing atomic radius.

On the other hand, as you move down a group, additional electron layers are added. These extra layers increase the shielding effect, causing the outer electrons to be less attracted to the nucleus, and thus the atomic radius increases.

In terms of ionisation energy, the greater the shielding effect, the less energy is required to remove an electron from an atom, as the electron is less strongly attracted to the nucleus. Conversely, a lower shielding effect means a higher ionisation energy.

Electron affinity and electronegativity also depend on the shielding effect. Atoms with a lower shielding effect have a higher electron affinity and electronegativity because the outer electrons are more strongly attracted to the nucleus.

In summary, electron shielding is a fundamental concept that helps explain the trends observed in the periodic table. Understanding this concept can provide a deeper insight into the behaviour of atoms and their interactions.