Q.Explain the screening effect with a suitable example.

Step 1. Define the screening effect. In a multi-electron atom, inner-shell electrons repel outer-shell electrons, partially cancelling the nucleus's pull on them; the outer electron therefore experiences a net EFFECTIVE nuclear charge $Z_{eff} = Z - \sigma$ that is smaller than the actual nuclear charge Z, where $\sigma$ is the shielding (screening) constant contributed by the inner/core electrons (section 7.5.1).

Step 2. Choose a worked example: sodium. Na has $Z=11$ and configuration $[Ne]3s^1 = 1s^22s^22p^63s^1$. Its 10 core electrons ($1s^22s^22p^6$) sit between the nucleus and the single 3s valence electron.

Step 3. Estimate the shielding. Each of the 10 core electrons shields roughly one unit of nuclear charge from the 3s electron, so $\sigma \approx 10$ and $Z_{eff} \approx 11-10 = 1$ — the 3s electron feels an effective pull close to just +1, even though the true nuclear charge is +11. …

Defining the screening effect and illustrating it quantitatively with sodium …

• Confusing the screening effect with electronegativity or ionization enthalpy themselves, rather than identifying it as the underlying CAUSE of their trends. …