Bi (V) represents the +5-oxidation state of Bismuth (Bi). It means Bi has lost 5 electrons from its valence shell. Bi, therefore, wants to gain the lost electrons and is called an oxidizing agent. The process of electron uptake is a reduction reaction.
Bi (V) is a stronger oxidizing agent than Sb (V) and, therefore, will pick up the electrons faster than Sb (V). Due to the electron uptake, it will then go from a +5-oxidation state to a +3-oxidation state. Bi is now less by only three electrons due to the two-electron gain.
Bi is placed below Sb in the periodic table, and the strength of the oxidizing agent decreases down the group from N, P, As, Sb to Bi. Thus, the oxidizing strength of Bi should be less than Sb, but it is observed to be the reverse.
The electronic configuration of Sb and Bi is-
Sb- [Kr] 4d10 5s2 5p3
Bi- [Xe] 4f14 5d10 6s2 6p3
Expanding the Bi configuration to compare with the Sb configuration, we see that Bi has additional d and f orbitals.
Bi -[Kr] 4d10 5s2 5p64f14 5d10 6s2 6p3
The d and f orbitals are the diffused orbitals. They do not provide a cover or a shield to the outer 6s orbitals electrons from the nuclear pull. As a result, the s-orbital electrons are tightly held by the attractive nuclear force, and they do not participate in any bonding reactions. Therefore, the two electrons of the 6s2 form an inert pair, and the condition is known as the ‘inert pair effect.’
Since only Bi has the d and f orbitals, the inert pair effect is confined to only Bi and not Sb.
In the +5-oxidation state of Bi, its electronic configuration is,
Bi (+5) - [Kr] 4d10 5s2 5p6 4f14 5d10 6s0 6p0
The electrons from the outer 6s and 6 p are lost. Therefore, in a reduction reaction to gain the two electrons, Bi is stronger than Sb as an oxidizing agent to fill up the vacant 6s orbitals, its inert pair. Bi then achieves the stable +3 oxidation state.
Bi (+3) - [Kr] 4d10 5s2 5p6 4f14 5d10 6s2 6p0
The +3-oxidation state is the most stable state for Bi. Bi's only known stable +5 oxidation state is in BiF5 and could be due to the small size and higher electronegativity of Fluorine. The +5-oxidation state of Bi with other halogens does not exist.
Therefore, due to the inert pair effect of Bi and not Sb, Bi (V) is a stronger oxidizing agent than Sb (V). Bi gains electrons to reach a stable +3 oxidation state.
Tip:
Mnemonic to remember the Group 15 Elements-
Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), Bismuth (Bi).
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