Defining Effective Nuclear Charge
Effective nuclear charge (Zeff) is the net positive charge experienced by a single electron in a multi-electron atom. It is less than the actual nuclear charge (the number of protons) because of the shielding effect of other electrons, especially inner-shell electrons.
The Role of Electron Shielding
Inner-shell electrons partially block the attraction between the nucleus and outer-shell (valence) electrons. This 'shielding' reduces the full positive charge of the nucleus that the valence electrons would otherwise experience. The effective nuclear charge can be approximated by the formula Zeff = Z - S, where Z is the atomic number (number of protons) and S is the shielding constant, often estimated by the number of core electrons.
Effective Nuclear Charge in Action
Consider Lithium (Li) with 3 protons. Its two core electrons shield the single valence electron, so the valence electron experiences a Zeff of approximately +1 (3-2). For Neon (Ne) with 10 protons, it also has two core electrons, but its eight valence electrons experience a much stronger Zeff of about +8 (10-2), because the nuclear charge has increased while the shielding from core electrons remains constant.
Importance in Periodic Trends
Effective nuclear charge is fundamental to explaining periodic trends. A higher Zeff results in a stronger pull on valence electrons, leading to a smaller atomic radius across a period. It also increases ionization energy, making it harder to remove an electron, and enhances electronegativity, increasing an atom's tendency to attract electrons.