4_1 Group 15 (VA) Elements: Trends and the Fixation of Nitrogen

Group 15 (VA) contains nitrogen, phosphorus, arsenic, antimony and bismuth. Each atom has five valence electrons,

ns^2np^3

, with the three p electrons unpaired. This half-filled p subshell gives the group its typical oxidation states of −3, +3 and +5. Like group 14, the group runs from non-metals at the top to a metal at the bottom. Nitrogen also has a special place in biology, because every protein and nucleic acid depends on it.

Trends Down the Group

Group 15 elements: selected properties

Property	N	P	As	Sb	Bi
Configuration	[He]2s²2p³	[Ne]3s²3p³	[Ar]3d¹⁰4s²4p³	[Kr]4d¹⁰5s²5p³	[Xe]4f¹⁴5d¹⁰6s²6p³
1st ionization energy (kJ/mol)	1402	1012	947	834	703
Electronegativity	3.0	2.2	2.2	2.1	2.0
Form at room temperature	Colourless gas (N₂)	White, red or black solid	Grey metalloid	Silvery metalloid	Pinkish-white metal
Character	Non-metal	Non-metal	Metalloid	Metalloid	Metal

Going down the group, atoms grow, ionization energy and electronegativity fall, and metallic character rises. Nitrogen and phosphorus form covalent compounds and acidic oxides. Arsenic and antimony are metalloids with amphoteric oxides, and bismuth is a metal whose oxide,

\mathrm{Bi_2O_3}

, is basic. The +3 state becomes more stable relative to +5 down the group, which is another example of the inert pair effect. Bismuth(V) compounds are therefore strong oxidizing agents.

Nitrogen differs sharply from the rest of the group. It is small and highly electronegative, it forms strong multiple bonds (N≡N, N=O) and it can take part in hydrogen bonding. Its maximum covalency is four, as in

\mathrm{NH_4^+}

, because only four orbitals are available in its second shell. Phosphorus and the heavier elements are larger, so they can bond to five neighbours, as in

\mathrm{PCl_5}

, which nitrogen can never do. Nitrogen exists as diatomic

\mathrm{N_2}

molecules, while phosphorus, arsenic and antimony form several solid allotropes, different structural forms of the same element.

Where Nitrogen Is Found

Free nitrogen makes up about 78% of air by volume. Nitrogen minerals are rare, because nitrates are very soluble and get washed away, apart from a few desert deposits of sodium nitrate. Combined nitrogen is concentrated instead in living things, in proteins, amino acids, nucleic acids and urea. The

\mathrm{N_2}

molecule is remarkably unreactive. Its triple bond needs about 945 kJ/mol to break, one of the strongest bonds known, and the molecule is non-polar. That is why it builds up in the atmosphere and why most organisms cannot use it directly.

Fixation of Nitrogen

Nitrogen fixation is any process that turns unreactive

\mathrm{N_2}

into compounds that plants can absorb, such as ammonium ions and nitrates. There are three main routes:

• Lightning: the intense heat of a lightning flash makes nitrogen and oxygen combine to form nitrogen monoxide,

  \mathrm{N_2 + O_2 \rightarrow 2NO}

  . This is oxidized further and dissolves in rain as nitric acid, which reaches the soil as nitrates.
• Biological fixation: bacteria such as Rhizobium, which live in the root nodules of legumes like peas, beans and clover, contain the enzyme nitrogenase. It reduces

  \mathrm{N_2}

  to ammonia at ordinary temperatures. This is why legumes enrich the soil.
• Industrial fixation: the Haber process makes ammonia from nitrogen and hydrogen, and most of it becomes fertilizer.

N#N.[H][H].[H][H].[H][H]>>N.N

The Haber reaction is reversible and exothermic. High pressure pushes the equilibrium towards ammonia, because 4 moles of gas become 2. A moderate temperature is a compromise between a good yield and a fast enough rate, and the iron catalyst speeds up breaking the stubborn triple bond. Fertilizers made from Haber ammonia now support food production for roughly half the world's population.

Fixed nitrogen does not stay fixed for ever. In the nitrogen cycle, soil bacteria oxidize ammonium to nitrite and then nitrate (nitrification). Plants absorb nitrate to build proteins, animals eat the plants, and decay returns ammonium to the soil. Finally, denitrifying bacteria convert nitrate back into

\mathrm{N_2}

, which closes the loop.

Tips for remembering

• N to Bi: non-metal to metal: nitrogen and phosphorus are non-metals, arsenic and antimony are metalloids, and bismuth is a metal.
• The triple bond makes N₂ lazy: N≡N needs 945 kJ/mol to break, so although air is mostly nitrogen, plants cannot use it directly.
• Three ways to fix nitrogen: lightning makes nitrogen oxides, bacteria make ammonia, and the Haber process makes ammonia industrially.