Effective nuclear charge (Zeff) represents the net positive charge experienced by an electron in a multi-electron atom. This value is less than the actual nuclear charge (Z) due to the shielding effect of other electrons. It is typically determined by subtracting the shielding constant (S) from the atomic number (Z). The shielding constant represents the extent to which other electrons in the atom reduce the nuclear charge felt by a particular electron. Mathematically, this is expressed as Zeff = Z – S. For instance, consider the outer electron of sodium (Na). While the nucleus contains 11 protons (Z = 11), the ten inner electrons shield the outer electron, resulting in an effective nuclear charge significantly lower than 11.
Understanding the effective nuclear charge is crucial in explaining various atomic properties, including ionization energy, atomic size, and electronegativity. A higher effective nuclear charge results in a stronger attraction between the nucleus and the valence electrons, leading to higher ionization energies and smaller atomic radii. The concept provides a valuable framework for rationalizing periodic trends and predicting the behavior of elements in chemical reactions. The approximation of nuclear shielding emerged as quantum mechanics developed to understand the behavior of multi-electron atoms.
