Group 15 Elements: The Nitrogen Family -- General Trends

Group 15, the nitrogen family, comprises nitrogen (N), phosphorus (P), arsenic (As),

antimony (Sb) and bismuth (Bi), all sharing the general outer-shell configuration $ns^2np^3$ --

a half-filled p sub-shell, which gives every member of this group an unusually high (locally

anomalous) first ionisation enthalpy compared with its immediate neighbours in Groups 14 and 16,

since a half-filled sub-shell is a position of extra stability.

Oxidation states in Group 15 range, in principle, from $-3$ (as in $\text{NH}_3$ or the

metal nitrides) through $0$ up to $+5$ (as in $\text{HNO}_3$ or $\text{H}_3\text{PO}_4$).

Nitrogen alone shows essentially every integral oxidation state in this range -- $-3$

($\text{NH}_3$), $-2$ ($\text{N}_2\text{H}_4$), $-1$ ($\text{NH}_2\text{OH}$), $0$ ($\text{N}_2$),

$+1$ ($\text{N}_2\text{O}$), $+2$ ($\text{NO}$), $+3$ ($\text{N}_2\text{O}_3$/$\text{HNO}_2$),

$+4$ ($\text{NO}_2$) and $+5$ ($\text{N}_2\text{O}_5$/$\text{HNO}_3$) -- a versatility unmatched

by any other Group 15 element. This is possible because nitrogen is small, has no available

$d$ orbitals to complicate its bonding, and can form strong $p\pi$-$p\pi$ multiple bonds with

itself and with oxygen, allowing it to satisfy its valency in many different combinatorial ways.

Phosphorus, arsenic, antimony and bismuth, by contrast, show mainly $+3$ and $+5$, following the

same inert pair effect seen in Groups 13 and 14: as one descends the group, the $+3$ state

(using only the three $np$ electrons, with the $ns^2$ pair left non-bonding) becomes

progressively favoured over the group oxidation state $+5$, culminating in bismuth, whose $+5$

compounds (e.g. $\text{NaBiO}_3$) are strong oxidising agents that are readily reduced back to

the more stable $\text{Bi}^{3+}$.

Nitrogen's anomalous behaviour relative to the rest of the group mirrors the pattern already

seen for boron and carbon at the head of Groups 13 and 14. Its very small atomic radius (about

70 pm, versus roughly 110 pm for phosphorus) and correspondingly high electronegativity and

ionisation enthalpy give nitrogen an essentially exclusively covalent chemistry with a strong

tendency to form multiple bonds (as in $\text{N}\equiv\text{N}$ or $\text{N}=\text{O}$).

Phosphorus and the heavier members, in contrast, show a much weaker tendency to multiple-bond and

instead favour extended catenated or ringed structures (as seen in the allotropes of phosphorus

discussed later in this chapter) or simple single-bonded compounds with expanded coordination

made possible by their accessible $d$ orbitals.

Across the group, atomic and ionic radius increases steadily down the group as successive

shells are added; ionisation enthalpy decreases overall (again with the characteristic small …