Effective Nuclear Charge ($Z_{eff}$) is the actual net positive charge experienced by an electron in a multi-electron atom.

• Across a period (left to right), the number of protons (atomic number $Z$) increases by one at each step.
• Simultaneously, electrons are added to the same valence shell.
• Since the shielding provided by electrons in the same shell is not perfect, the gain in $Z$ outweighs the gain in shielding $(\sigma )$.
• Result: $Z_{eff}$ increases across a period, pulling electrons tighter and reducing atomic radius.