Group 15 — Nitrogen Family: General Characteristics

Members and Occurrence

Group 15 comprises nitrogen, phosphorus, arsenic, antimony, bismuth and the synthetic radioactive element moscovium (Mc). The defining narrative of the group is a steady shift from non-metallic to metallic character: nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is a typical metal. Because moscovium has a very short half-life and is available only in trace amounts, its chemistry is not yet established, so it is set aside in NEET-level study.

The occurrence of the two lightest members reflects their biological importance. Molecular nitrogen makes up about 78% by volume of the atmosphere, and in the earth's crust it occurs as sodium nitrate $\ce{NaNO3}$ (Chile saltpetre) and potassium nitrate (Indian saltpetre); it is also bound in the proteins of plants and animals. Phosphorus occurs in minerals of the apatite family such as fluorapatite $\ce{Ca9(PO4)6.CaF2}$, the main component of phosphate rocks, and is an essential constituent of bones and living cells. Arsenic, antimony and bismuth occur mainly as sulphide minerals.

Electronic Configuration

The valence-shell electronic configuration of every Group 15 element is $ns^2np^3$. The $s$ orbital is completely filled and the three $p$ orbitals are each singly occupied — a half-filled $p$ subshell. This half-filled arrangement carries extra exchange-energy stability, and the consequences of that stability ripple through almost every trend in the group, most visibly in the unusually high ionisation enthalpy.

Atomic and Physical Trends

Covalent and ionic radii increase down the group, but not uniformly. There is a considerable jump in covalent radius from N to P; from As to Bi, however, only a small increase is observed because the heavier members carry completely filled $d$ and/or $f$ orbitals that screen the nuclear charge poorly, pulling the outer shell inward.

Ionisation enthalpy decreases down the group as atomic size grows. Two points carry NEET weight. First, owing to the extra-stable half-filled $p$ configuration and smaller size, the ionisation enthalpy of Group 15 elements is markedly greater than that of the Group 14 elements in the same period. Second, successive ionisation enthalpies follow the expected order $\Delta_iH_1 < \Delta_iH_2 < \Delta_iH_3$.

Electronegativity generally decreases down the group with increasing atomic size, though among the heavier elements the difference is not pronounced (As, Sb and Bi cluster near 1.9–2.0). Metallic character increases down the group, a direct consequence of the falling ionisation enthalpy and rising atomic size. All Group 15 elements are polyatomic: dinitrogen is a diatomic gas while the rest are solids. Boiling points generally rise down the group, whereas the melting point rises up to arsenic and then falls towards bismuth.

Oxidation States

The common oxidation states of Group 15 elements are −3, +3 and +5. Two opposing trends govern them. The tendency to show the −3 state decreases down the group as size and metallic character grow — bismuth scarcely forms any −3 compound. The stability of the +5 state also decreases down the group, while the +3 state becomes more stable, owing to the inert pair effect: the $ns^2$ pair becomes increasingly reluctant to participate in bonding. The only well-characterised Bi(V) compound is $\ce{BiF5}$.

Nitrogen is exceptionally versatile with oxygen, additionally showing +1, +2 and +4 states (as in its various oxides). It does not, however, form compounds in the +5 state with halogens, because it lacks $d$ orbitals to accommodate the extra bonding electrons. Phosphorus also shows +1 and +4 states in some oxoacids.

A signature reactivity pattern is disproportionation. For nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution, for example:

$$\ce{3HNO2 -> HNO3 + H2O + 2NO}$$

Nearly all intermediate oxidation states of phosphorus disproportionate into +5 and −3, in both alkali and acid. For arsenic, antimony and bismuth, by contrast, the +3 state becomes increasingly stable against disproportionation down the group.

Anomalous Behaviour of Nitrogen

Nitrogen differs sharply from the rest of its group because of four linked features: small size, high electronegativity, high ionisation enthalpy and the absence of $d$ orbitals. The most consequential effect is nitrogen's unique ability to form $p\pi$–$p\pi$ multiple bonds with itself and with other small, electronegative atoms such as carbon and oxygen. Heavier Group 15 atoms have orbitals so large and diffuse that effective $p\pi$–$p\pi$ overlap is not possible.

Consequently, nitrogen exists as a diatomic molecule with a triple bond — one $\sigma$ and two $\pi$ — and a very high bond enthalpy of 941.4 kJ mol⁻¹:

$$\ce{N#N} \qquad (\Delta_{diss}H = 941.4~\text{kJ mol}^{-1})$$

Phosphorus, arsenic and antimony instead form single bonds (P–P, As–As, Sb–Sb), and bismuth forms metallic bonds in the elemental state. Curiously, the single N–N bond is weaker than the single P–P bond, because the short N–N bond length forces strong interelectronic repulsion between the non-bonding lone pairs. This weak single bond is exactly why nitrogen has a poor catenation tendency compared with phosphorus.

The absence of $d$ orbitals also means nitrogen cannot form $d\pi$–$p\pi$ bonds, which the heavier members manage in species such as $\ce{R3P=O}$ and $\ce{R3P=CH2}$. Phosphorus and arsenic can even form $d\pi$–$d\pi$ bonds with transition metals when ligands like $\ce{P(C2H5)3}$ and $\ce{As(C6H5)3}$ coordinate.

Hydrides of Group 15

Every Group 15 element forms a hydride of the type $\ce{EH3}$ — $\ce{NH3}$, $\ce{PH3}$, $\ce{AsH3}$, $\ce{SbH3}$ and $\ce{BiH3}$ — and these show a remarkably regular gradation. Stability, measured by the E–H bond dissociation enthalpy, decreases from $\ce{NH3}$ to $\ce{BiH3}$. As the bond weakens, the hydride gives up hydrogen more readily, so reducing character increases down the series: ammonia is only a mild reducing agent whereas $\ce{BiH3}$ is the strongest reducing agent among them. Basicity falls in the order:

$$\ce{NH3 > PH3 > AsH3 > SbH3 > BiH3}$$

Two further trends matter. The bond angle (H–E–H) decreases from 107.8° in $\ce{NH3}$ to roughly 91° by $\ce{SbH3}$, as the bonding shifts toward almost pure $p$ orbitals down the group. Boiling points show an anomaly at the top: $\ce{NH3}$ has a higher melting and boiling point than $\ce{PH3}$ because the small, highly electronegative nitrogen lets $\ce{NH3}$ associate through intermolecular hydrogen bonding in the solid and liquid states.

Oxides and Oxoacids

Each Group 15 element forms two principal oxide types, $\ce{E2O3}$ and $\ce{E2O5}$, corresponding to the +3 and +5 states. The oxide in the higher oxidation state is more acidic than that in the lower state, and acidic character of the oxides decreases down the group. The $\ce{E2O3}$ oxides trace the metal–non-metal trend precisely: those of nitrogen and phosphorus are purely acidic, arsenic and antimony are amphoteric, and bismuth is predominantly basic.

Reactivity toward halogens parallels this: the elements form $\ce{EX3}$ and $\ce{EX5}$ halides, except that nitrogen forms no pentahalide. Pentahalides are more covalent than trihalides because the element in the +5 state has greater polarising power than in the +3 state. All trihalides except those of nitrogen are stable. The corresponding oxoacids — $\ce{HNO2}$, $\ce{HNO3}$ and the family of phosphorus oxoacids — are treated separately in the sibling notes.

Allotropy

Except nitrogen, all the elements of Group 15 show allotropy. Nitrogen exists only as the diatomic $\ce{N2}$ molecule; its strong $p\pi$–$p\pi$ triple bond leaves no room for the structural variation seen further down. Phosphorus is the textbook case, with white, red and black allotropes built from $\ce{P4}$ tetrahedra and chains. Arsenic and antimony display grey (metallic) and yellow forms, and the metallic grey allotrope dominates as one moves toward bismuth.

The progression from a single molecular form (N) to multiple solid allotropes mirrors the rising metallic character of the group: the heavier the element, the more readily it adopts extended, metal-like solid structures.