What is inert pair effect?

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Q. What is inert pair effect?

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Concept understanding — Group 15 Elements — Occurrence and Physical Properties

Group 15 (the pnictogens/nitrogen family) consists of nitrogen, phosphorus, arsenic, antimony and bismuth, all sharing the outer configuration ns2 np3. Nitrogen, about 78% of the atmosphere by volume, also occurs combined as Chile saltpetre (NaNO3) and Indian saltpetre (KNO3). Phosphorus, the 11th most abundant crustal element, is never free, occurring instead as fluorapatite, chlorapatite and hydroxyapatite. Arsenic, antimony and bismuth are comparatively scarce and occur mainly as sulphide ores.

Down the group, atomic radius, atomic mass and (generally) melting/boiling point rise, while nitrogen alone is a gas and the rest are solids; density rises sharply from nitrogen's near-negligible gas density to bismuth's 9.79 g/cm3. Nitrogen's exceptionally stable half-filled 2p3 configuration gives it a high first ionisation enthalpy and the third-highest electronegativity of any element, and, combined with its small size and lack of accessible d orbitals, this makes nitrogen chemically anomalous within its own group: it strongly prefers pπ-pπ multiple bonding with itself (N≡N) and cannot expand its octet the way phosphorus and heavier members can (which instead favour catenated single bonds, as in P4, and readily form 5-coordinate species like PCl5). This same small size/high electronegativity/no-d-orbitals trio also underlies the classic "inert pair effect" seen most strongly at the bottom of the group: bismuth's 6s2 electron pair is reluctant to participate in bonding, making Bi3+ markedly more stable than Bi5+ (the reverse of the trend at the top of the group, where nitrogen and phosphorus favour the group oxidation state +5).

The valence-shell configuration ns2 np3 is common to every member (N: 2s2 2p3; P: 3s2 3p3; As: 4s2 4p3; Sb: 5s2 5p3; Bi: 6s2 6p3), and it is this half-filled p subshell that gives the group its characteristic +3/+5 oxidation-state pair (three unpaired p electrons for +3 covalency/oxidation, or all five valence electrons for +5 after promotion, where d orbitals or larger size permit it).

Step 1. State the definition.

The inert pair effect is the tendency of the pair of s electrons in the outermost (valence) shell of the heavier elements of a p-block group to remain non-bonding ("inert"), rather than being promoted and used in bond formation along with the p electrons.

Step 2. Explain the underlying cause.

As one goes down a p-block group, poor shielding by the intervening filled d (and f) subshells lets the ns electrons experience an unusually high effective nuclear charge, holding them more tightly than a simple size-based extrapolation would suggest; the ns-np energy gap also widens down the group, making promotion of the ns2 pair into bonding increasingly costly in energy.

Step 3. State the consequence for oxidation states.

Because this ns2 pair resists bonding, the heaviest members of a p-block group increasingly favour an oxidation state two units below the group's characteristic maximum oxidation state, using only their np electrons for bonding and leaving the ns2 pair as a stereochemically inert lone pair.

Step 4. Give the Group 15 example from this unit.

In Group 15, this is why bismuth's +3 state (Bi3+, from the 6p3 electrons alone) is markedly more stable than its +5 state, in contrast to nitrogen and phosphorus, which readily show the full +5 group oxidation state.

✓Final answer

The inert pair effect is the reduced tendency of a heavy p-block element's outer ns2 electrons to participate in bonding, causing the (group oxidation state - 2) state — such as Bi3+ over Bi5+ in Group 15 — to become the more stable oxidation state.

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Q.What is inert pair effect?

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