Q.The stability of +1 oxidation state increases in the sequence

Step 1. The inert pair effect (Section 2.1.6) makes the ns² electron pair increasingly reluctant to take part in bonding as group 13 is descended, favouring the +1 state (two less than the +3 group state) more and more strongly for the heavier elements.

Step 2. At the top of the group, Al³⁺ is more stable than Al⁺ — i.e. +1 is the LEAST favoured state for aluminium among the five elements.

Step 3. At the bottom of the group, the chapter states explicitly that Tl⁺ is MORE stable than Tl³⁺ — i.e. +1 is the MOST favoured, most stable state for thallium of the five. …

Apply the inert-pair-effect trend of Section 2.1.6 (growing preference for the +1 state on descending group 13) directly to order th …

• Reversing the direction of the trend (assuming +1 stability FALLS down the group, as many other periodic trends do) — the inert pair effect works the opposite way: it GROWS down the gr …