Oxidation States & Reactivity Trends
The Inert Pair Effect, N-N single bonds, and restricted covalency.
Group 15 elements have five electrons in their valence shell ($ns^2 np^3$). They can either gain 3 electrons to achieve an octet or share/lose up to 5 electrons. This flexibility results in a wide variety of oxidation states, but their stability varies drastically as you move down the group due to atomic size and orbital shielding.
1. Common Oxidation States (-3, +3, +5)
A. The Negative State: -3
To show a -3 oxidation state, the atom must be small and highly electronegative to attract 3 electrons.
- Trend: The tendency to exhibit a -3 oxidation state decreases rapidly down the group due to increasing atomic size and increasing metallic character.
- Nitrogen: Readily shows -3 in compounds like Ammonia ($NH_3$) and nitrides ($Mg_3N_2$).
- Bismuth: Hardly forms any compound in the -3 oxidation state.
B. The Positive States (+3 and +5) & The Inert Pair Effect
Group 15 elements can utilize their $p$ electrons (+3) or both their $s$ and $p$ electrons (+5) in bonding. Here lies one of the most frequently tested trends in the p-block.
- The stability of the +5 oxidation state decreases.
- The stability of the +3 oxidation state increases.
Why? The Inert Pair Effect
As you move down the group (especially from As to Bi), the intervening $d$ and $f$ orbitals are filled. These orbitals provide very poor shielding of the nuclear charge. The nucleus pulls strongly on the $ns^2$ electrons, holding them tightly and making them reluctant to participate in bond formation. Thus, the $ns^2$ pair becomes "inert," and only the three $np^3$ electrons are easily available, making the +3 state highly stable for Bismuth.
Figure 1: Opposing stability trends of the +3 and +5 oxidation states down Group 15.
2. Anomalous Properties of Nitrogen
Like the first member of every p-block group, Nitrogen differs drastically from the rest of its family. This is due to its small size, high electronegativity, high ionization enthalpy, and non-availability of d-orbitals.
A. $p\pi - p\pi$ Multiple Bonding
Nitrogen has the unique ability to form $p\pi - p\pi$ multiple bonds with itself and with other elements having small size and high electronegativity (like C, O). Thus, Nitrogen exists as a diatomic gas ($N \equiv N$).
Heavier elements of this group (like P, As) cannot form effective $p\pi - p\pi$ bonds because their atomic orbitals are too large and diffuse to have effective overlapping. Phosphorus exists as a solid $P_4$ molecule with single bonds.
B. The Weak N-N Single Bond & Catenation
While the $N \equiv N$ triple bond is exceptionally strong, the $N-N$ single bond is weaker than the $P-P$ single bond.
C. Restricted Covalency (Why $NCl_5$ doesn't exist)
Nitrogen belongs to the second period, so its valence shell ($n=2$) contains only $2s$ and $2p$ orbitals. It completely lacks vacant d-orbitals.
- The maximum covalency of Nitrogen is restricted to 4 (one s and three p orbitals), for example in the Ammonium ion ($NH_4^+$).
- It cannot expand its octet. Therefore, while Phosphorus can form $PCl_5$ (by utilizing its 3d orbitals), Nitrogen cannot form $NCl_5$.
D. Disproportionation Reactions
Nitrogen exhibits all oxidation states from -3 to +5. In acid solution, almost all oxidation states from +1 to +4 tend to disproportionate (simultaneously oxidize and reduce).
($N$ goes from +3 in $HNO_2$ to +5 in $HNO_3$ and +2 in $NO$)
3. Reactivity Trends: Hydrides
All Group 15 elements form hydrides of the type $EH_3$ (like $NH_3, PH_3, AsH_3, SbH_3, BiH_3$). Down the group:
- Bond dissociation enthalpy decreases: Size of E increases, so the E-H bond becomes longer and weaker.
- Thermal Stability decreases: Because the bond is weaker, $BiH_3$ is the least stable.
- Reducing Character increases: The E-H bond breaks easier, releasing Hydrogen, making $BiH_3$ the strongest reducing agent.
- Basic Character decreases: $NH_3$ is the most basic. As you go down, the electron density of the lone pair is dispersed over a much larger volume, making it less available for donation.
Mastery Check: Oxidation States & Reactivity
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