Chemistry · Ch 9 — Elements of Group 13, 14 and 15
Oxidation state
Oxidation state
Oxidation state is the primary chemical property used to classify how these elements bond. The highest oxidation state shown by any p-block element equals the total number of valence electrons (s-electrons plus p-electrons); this is called the group oxidation state — +3 for group 13, +4 for group 14 and +5 for group 15. For the lighter members of each family (boron, carbon, nitrogen and their nearest neighbours) this group oxidation state is also the most stable one. Every element in these three groups, however, can additionally show an oxidation state two units lower than the group oxidation state: +1 for group 13, +2 (or −4) for group 14, and +3 (or −3) for group 15. Down each group, this lower oxidation state becomes increasingly stable relative to the group oxidation state, so that for the heaviest members (thallium, lead, bismuth) it is actually the more stable one — see Table 9.5 for the full set of oxidation states and example compounds. This increased stability of the state lowered by two units in heavier p-block elements is called the inert-pair effect: the outer ns² electron pair becomes progressively less willing to take part in bonding, because it is poorly shielded by the ten …
Group 13 — outer configuration ns²np¹, group oxidation state +3, other oxidation state +1, example stable compounds BF3, AlCl3, GaCl3, InCl3, TlBr. Group 14 — outer configuration ns²np², group oxidation state +4, other oxidation states +2 and -4, example stable compounds CH4, CO2, CCl4, SiCl4, GeCl4, SnCl2, PbCl2. Group 15 — outer configuration ns²np³, group oxidation state +5, other oxidation states +3 and -3, example stable compounds N2O5, NH3, NF3, PH3, PCl5, AsH3, SbH3, SbCl3, BiCl3. Note that TlBr, SnCl2/PbCl2 and BiCl3/SbCl3 are all ex …
Worked out. Worked problem: why is the Tl+1 ion more stable than Tl+3? Solution: thallium is a heavy element of group 13, whose group (highest) oxidation state is +3. In the p-block, the oxidation state lowered by two units from the group oxidation state becomes progressively more stable for heavier elements — the inert-pair effect — because the outer ns2 electron pair is shielded poorly by the intervening d (and, for Tl, f) electrons and so resists taking part in further bonding. Since thallium is one of the heaviest group 13 elements, this inert-pair effect is very pronounced for it, which is why th …