Question

Use the idea of effective nuclear charge, Z_eff = Z – σ, to explain the decrease in atomic size across a row. What exactly does this shielding term (σ) represent? (Please answer in a simplified manner and then scientific manner)

Answer 1

Across a row, the atomic nmber increased by one for each succeeding element. The valence shell electrons, being in the same shell aross a row. So, they shield each other poorly from the increasing nuclear charge.. So, the contribution of sigma to effictive nuclear charge is negligible. Thus Z_ef increases as atomic number (Z) increases.

Across a row, the principal quantum number stays constant, so that whether we use Z_eff or just the nuclear charge Z, the result is the atomic size decreases across a row.