Effective Nuclear Charge

Effective nuclear charge is the net positive charge actually felt by a valence electron after accounting for the screening by other electrons. Across a period, each added electron enters the same outermost shell, and electrons in the same shell shield one another rather poorly compared with inner-shell electrons. Meanwhile each step adds a proton to the nucleus, steadily increasing the actual nuclear charge. Because the increase in nuclear charge outweighs the weak same-shell shielding, the effective nuclear charge felt by valence electrons rises across the period. The option that inner electrons increase rapidly is wrong, since inner shells stay the same within a period. The option that the nucleus loses protons is factually impossible across a period. The option that atomic radius increases contradicts the actual contraction. This concept, often estimated by Slater's rules, underlies many NCERT periodic trends. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. This concept also bridges to Atomic Structure and Chemical Bonding and Molecular Structure, so mastering it strengthens performance on linked questions from those topics as well. Plausibility check: the rising effective nuclear charge correctly explains the decreasing radius and rising ionization enthalpy across a period, confirming the reasoning.