The nucleus of an atom contains protons, which carry positive charges, while electrons orbit around the nucleus in energy levels or shells. According to the principles of electrostatic attraction, the positively charged nucleus exerts an attractive force on the negatively charged electrons.

However, the presence of inner electrons between the nucleus and the outer electrons creates a repulsive force that shields the outer electrons from experiencing the full strength of the attractive force of the nucleus. This shielding effect arises due to electron-electron repulsions.

Effective nuclear charge (Zeff) is a measure of the net positive charge experienced by an electron in an atom. It takes into account both the attractive force from the nucleus and the repulsive forces from other electrons. The effective nuclear charge experienced by an electron is less than the actual nuclear charge because of the shielding effect.

The concept of shielding and Zeff has important implications for understanding atomic properties, such as atomic size, ionization energy, and electron affinity. Higher levels of shielding result in a reduced attraction between the outermost electrons and the nucleus, leading to larger atomic size and lower ionization energy. Conversely, lower levels of shielding result in a stronger attraction, leading to smaller atomic size and higher ionization energy.

Additionally, the shielding effect also influences the ability of an atom to attract and hold additional electrons. This concept is especially relevant in understanding chemical bonding and the formation of ions.

In the periodic table, the trend in Zeff and shielding across a period is crucial. As we move from left to right across a period, the number of protons in the nucleus increases, leading to a greater positive charge in the nucleus. However, the shielding effect remains relatively constant as electrons are added to the same energy level or shell. As a result, the effective nuclear charge experienced by the outermost electrons increases, leading to a stronger attraction and smaller atomic size.