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IN p-block 13 group why oxidation state +5 decreases down the group

Due to small size of boron, the sum of its first three ionization enthalpies is very high. This prevents it from +3 ions and forces it to form only covalent compounds. But as we move from B to Al , the sum of the first three ionization enthalpies of Al considerably decreases and is therefore able to form Al3+ ions. In fact Al is highly electropositive metal. However, down the group , due to poor shielding effect of intervening d and f orbitals, the increased effective nuclear charge holds n s electrons tightly (responsible for inert pair effect ) and thereby , restricting their participation in bonding. As a result of this only p-orbital electron may be involved in bonding. In fact in Ga , In and Tl , both +1 and +3 oxidation states are observed. The relative stability of +1 oxidation state progressively increases for heavier elements : Al < Ga < In < Tl . In thallium +1 oxidation state is predominant whereas the +3 oxidation state is highly oxidizing in character. The compounds in +1 oxidation state , as expected from energy considerations , are more ionic than those in +3 oxidation state.


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