The effective nuclear charge (often denoted as ZeffZ_{\text{eff}}Zeff) is the net positive charge experienced by an electron in a multi-electron atom. It reflects the actual attractive force that an electron feels from the nucleus after accounting for the repulsion or shielding effect caused by other electrons between the nucleus and the electron in question. The shielding reduces the full nuclear charge felt by outer electrons. Mathematically, it is approximated by:
Zeff=Z−SZ_{\text{eff}}=Z-SZeff=Z−S
where ZZZ is the atomic number (number of protons in the nucleus), and SSS is the shielding constant representing the number of inner (core) electrons that reduce the nuclear charge felt by the outer electron. This concept is crucial for understanding atomic structure, periodic trends, and properties like ionization energy. Electrons closer to the nucleus experience a higher effective nuclear charge, while those farther away feel less due to greater shielding by inner electrons. Effective nuclear charge generally increases across a period in the periodic table and decreases down a group due to these shielding effects.
