Q.Explain the trends in atomic radii of d block elements

Step 1. State the trend. Moving from left to right across any first-row transition series (e.g. Sc to Zn), atomic radius decreases gradually (Fig 8.3, Table 8.5) -- from Sc at 164 pm down to a broadly smaller value by the end of the row.

Step 2. Explain the underlying cause. Each step to the right adds one more proton to the nucleus (nuclear charge rises by one unit at a time) AND one more electron, but that new electron always enters the SAME, already partly-occupied (n-1)d subshell rather than starting a fresh outer shell. Because d orbitals are less penetrating (more spatially diffuse/spread out) than s or p orbitals of the same shell, d electrons shield the growing nuclear charge from each other relatively poorly.

Step 3. Connect cause to effect. Because shielding lags behind the steadily-rising actual nuclear charge, the EFFECTIVE nuclear charge felt by the outer electrons also keeps climbing across the series, pulling the electron cloud in progressively closer to the nucleus at every step -- this is what produces the observed gradual shrinkage in atomic radius.

Step 4. Note the trend's limits. The chapter explicitly states that explaining the small element-to-element irregularities WITHIN this overall downward trend is beyond its scope -- only the smooth, overall left-to-right decrease is expected to be explained.

Step 5. Extend to ionic radii. Ionic radii follow the identical logic and show the same overall trend (Table 8.5): for a FIXED oxidation state, ionic radius decreases across the series (most cleanly seen for the divalent M2+ ions, e.g. Cr2+ 82 pm vs Cu2+ 73 pm); and for a GIVEN element, ionic radius decreases from a lower to a higher oxidation state (M2+ to M3+), since removing more electrons raises the effective nuclear charge felt by those remaining.

Explaining Section 8.6.1's atomic/ionic radius trend via the nuclear-charge-vs-shielding balance, extended to both fixed-oxidation-state and variable-oxidation-state ionic radius comparisons

• Explaining the trend purely in terms of 'more protons = smaller radius' without mentioning the poor SHIELDING by d electrons as the specific reason the trend continues smoothly (unlike, say, a full new shell being added).
• Forgetting to address the ionic-radius half of the trend (both same-oxidation-state comparison across the row and different-oxidation-state comparison for one element), which the full syllabus answer expects alongside the atomic-radius trend.