Q.Explain why is there a phenomenal decrease in ionization enthalpy from carbon to silicon?

Carbon's valence electrons are in the small $n=2$ shell, close to the nucleus with minimal inner shielding, so they are held tightly (high ionization enthalpy). Silicon's valence electrons are in the much larger $n=3$ shell, screened by a complete inner shell of 10 electrons ($1s^22s^22p^6$), so the effective nuclear charge felt i …

1. Position in the periodic table. Carbon is in period 2 (electronic configuration $[\text{He}]2s^22p^2$), while silicon is directly below it in period 3 ($[\text{Ne}]3s^23p^2$).
2. Change in atomic size. Moving from carbon to silicon adds an entire new principal quantum shell ($n=2 \to n=3$), which sharply increases the atomic radius (carbon $\approx 77$ pm vs silicon $\approx 118$ pm).
3. Change in shielding. In silicon, the $3s3p$ valence electrons are shielded from the nucleus by a complete inner shell of 10 electrons ($1s^22s^22p^6$), which is a very effective shield since it is a full, symmetric shell — unlike carbon's valence electrons, which are shielded only by the small, tightly-held $1s^2$ core.
4. Effect on effective nuclear charge. The combination of much greater distance and much better shielding means the effective nuclear charge experienced by silicon's valence electron is considerably lower than that experienced by carbon's. …

Compare the electronic configurations and atomic radii of C and Si, identify that Si has an additional complete quantum shell providing much stronger shielding, and connect increased size + shielding to reduced effective nuclear char …

• Attributing the drop only to "increase in atomic size" without also mentioning the crucial role of increased shielding by the added inner shell. …