The Elements of Group 15 (The Pnicogens)

22.3 The Elements of Group 15 (The Pnicogens)

Learning Objective

To understand the trends in properties and reactivity of the group 14 elements: the pnicogens.

Like the group 14 elements, the lightest member of group 15, nitrogen, is found in nature as the free element, and the heaviest elements have been known for centuries because they are easily isolated from their ores.

Antimony (Sb) was probably the first of the pnicogens to be obtained in elemental form and recognized as an element. Its atomic symbol comes from its Roman name: stibium. It is found in stibnite (Sb₂S₃), a black mineral that has been used as a cosmetic (an early form of mascara) since biblical times, and it is easily reduced to the metal in a charcoal fire (Figure 22.10 "The Ancient Egyptians Used Finely Ground Antimony Sulfide for Eye Makeup"). The Egyptians used antimony to coat copper objects as early as the third millennium BC, and antimony is still used in alloys to improve the tonal quality of bells.

Figure 22.10 The Ancient Egyptians Used Finely Ground Antimony Sulfide for Eye Makeup

(a) Crystals of the soft black mineral stibnite (Sb₂S₃) on a white mineral matrix. (b) A fragment of an Egyptian painting on limestone from the 16th–13th centuries BC shows the use of ground stibnite (“kohl”) as black eye shadow. Small vases of ground stibnite have been found among the funeral goods buried with Egyptian pharaohs.

In the form of its yellow sulfide ore, orpiment (As₂S₃), arsenic (As) has been known to physicians and professional assassins since ancient Greece, although elemental arsenic was not isolated until centuries later. The history of bismuth (Bi), in contrast, is more difficult to follow because early alchemists often confused it with other metals, such as lead, tin, antimony, and even silver (due to its slightly pinkish-white luster). Its name comes from the old German wismut, meaning “white metal.” Bismuth was finally isolated in the 15th century, and it was used to make movable type for printing shortly after the invention of the Gutenberg printing process in 1440. Bismuth is used in printing because it is one of the few substances known whose solid state is less dense than the liquid. Consequently, its alloys expand as they cool, filling a mold completely and producing crisp, clear letters for typesetting.

Phosphorus was discovered in 1669 by the German alchemist Hennig Brandt, who was looking for the “philosophers’ stone,” a mythical substance capable of converting base metals to silver or gold. Believing that human urine was the source of the key ingredient, Brandt obtained several dozen buckets of urine, which he allowed to putrefy. The urine was distilled to dryness at high temperature and then condensed; the last fumes were collected under water, giving a waxy white solid that had unusual properties. For example, it glowed in the dark and burst into flames when removed from the water. (Unfortunately for Brandt, however, it did not turn lead into gold.) The element was given its current name (from the Greek phos, meaning “light,” and phoros, meaning “bringing”) in the 17th century. For more than a century, the only way to obtain phosphorus was the distillation of urine, but in 1769 it was discovered that phosphorus could be obtained more easily from bones. During the 19th century, the demand for phosphorus for matches was so great that battlefields and paupers’ graveyards were systematically scavenged for bones. Early matches were pieces of wood coated with elemental phosphorus that were stored in an evacuated glass tube and ignited when the tube was broken (which could cause unfortunate accidents if the matches were kept in a pocket!).

Unfortunately, elemental phosphorus is volatile and highly toxic. It is absorbed by the teeth and destroys bone in the jaw, leading to a painful and fatal condition called “phossy jaw,” which for many years was accepted as an occupational hazard of working in the match industry.

Although nitrogen is the most abundant element in the atmosphere, it was the last of the pnicogens to be obtained in pure form. In 1772, Daniel Rutherford, working with Joseph Black (who discovered CO₂), noticed that a gas remained when CO₂ was removed from a combustion reaction. Antoine Lavoisier called the gas azote, meaning “no life,” because it did not support life. When it was discovered that the same element was also present in nitric acid and nitrate salts such as KNO₃ (nitre), it was named nitrogen. About 90% of the nitrogen produced today is used to provide an inert atmosphere for processes or reactions that are oxygen sensitive, such as the production of steel, petroleum refining, and the packaging of foods and pharmaceuticals.

Preparation and General Properties of the Group 15 Elements

Because the atmosphere contains several trillion tons of elemental nitrogen with a purity of about 80%, it is a huge source of nitrogen gas. Distillation of liquefied air yields nitrogen gas that is more than 99.99% pure, but small amounts of very pure nitrogen gas can be obtained from the thermal decomposition of sodium azide:

Equation 22.28

2 {NaN}_{3} (s) \overset{Δ}{\to} 2 Na(l) + 3 N_{2} (g)

In contrast, Earth’s crust is relatively poor in nitrogen. The only important nitrogen ores are large deposits of KNO₃ and NaNO₃ in the deserts of Chile and Russia, which were apparently formed when ancient alkaline lakes evaporated. Consequently, virtually all nitrogen compounds produced on an industrial scale use atmospheric nitrogen as the starting material. Phosphorus, which constitutes only about 0.1% of Earth’s crust, is much more abundant in ores than nitrogen. Like aluminum and silicon, phosphorus is always found in combination with oxygen, and large inputs of energy are required to isolate it.

The other three pnicogens are much less abundant: arsenic is found in Earth’s crust at a concentration of about 2 ppm, antimony is an order of magnitude less abundant, and bismuth is almost as rare as gold. All three elements have a high affinity for the chalcogens and are usually found as the sulfide ores (M₂S₃), often in combination with sulfides of other heavy elements, such as copper, silver, and lead. Hence a major source of antimony and bismuth is flue dust obtained by smelting the sulfide ores of the more abundant metals.

In group 15, as elsewhere in the p block, we see large differences between the lightest element (N) and its congeners in size, ionization energy, electron affinity, and electronegativity (Table 22.3 "Selected Properties of the Group 15 Elements"). The chemical behavior of the elements can be summarized rather simply: nitrogen and phosphorus behave chemically like nonmetals, arsenic and antimony behave like semimetals, and bismuth behaves like a metal. With their ns²np³ valence electron configurations, all form compounds by losing either the three np valence electrons to form the +3 oxidation state or the three np and the two ns valence electrons to give the +5 oxidation state, whose stability decreases smoothly from phosphorus to bismuth. In addition, the relatively large magnitude of the electron affinity of the lighter pnicogens enables them to form compounds in the −3 oxidation state (such as NH₃ and PH₃), in which three electrons are formally added to the neutral atom to give a filled np subshell. Nitrogen has the unusual ability to form compounds in nine different oxidation states, including −3, +3, and +5. Because neutral covalent compounds of the trivalent pnicogens contain a lone pair of electrons on the central atom, they tend to behave as Lewis bases.

Table 22.3 Selected Properties of the Group 15 Elements

Property	Nitrogen	Phosphorus	Arsenic	Antimony	Bismuth
atomic symbol	N	P	As	Sb	Bi
atomic number	7	15	33	51	83
atomic mass (amu)	14.01	30.97	74.92	121.76	209.98
valence electron configuration*	2s²2p³	3s²3p³	4s²4p³	5s²5p³	6s²6p³
melting point/boiling point (°C)	−210/−196	44.15/281^c	817 (at 3.70 MPa)/603 (sublimes)^†	631/1587	271/1564
density (g/cm³⁾ at 25°C	1.15 $(g/L)$	1.82^†	5.75^‡	6.68	9.79
atomic radius (pm)	56	98	114	133	143
first ionization energy (kJ/mol)	1402	1012	945	831	703
common oxidation state(s)	−3 to +5	+5, +3, −3	+5, +3	+5, +3	+3
ionic radius (pm)^§	146 (−3), 16 (+3)	212 (−3), 44 (+3)	58 (+3)	76 (+3), 60 (+5)	103 (+3)
electron affinity (kJ/mol)	0	−72	−78	−101	−91
electronegativity	3.0	2.2	2.2	2.1	1.9
standard reduction potential (E°, V) (E^V → E^III in acidic solution)^\|\|	+0.93	−0.28	+0.56	+0.65	—
product of reaction with O₂	NO₂, NO	P₄O₆, P₄O₁₀	As₄O₆	Sb₂O₅	Bi₂O₃
type of oxide	acidic (NO₂), neutral (NO, N₂O)	acidic	acidic	amphoteric	basic
product of reaction with N₂	—	none	none	none	none
product of reaction with X₂	none	PX₃, PX₅	AsF₅, AsX₃	SbF₅, SbCl₅, SbBr₃, SbI₃	BiF₅, BiX₃
product of reaction with H₂	none	none	none	none	none
*The configuration shown does not include filled d and f subshells.
^†For white phosphorus.
^‡For gray arsenic.
^§The values cited are for six-coordinate ions in the indicated oxidation states. The N⁵⁺, P⁵⁺, and As⁵⁺ ions are not known species.
^\|\|The chemical form of the elements in these oxidation states varies considerably. For N, the reaction is NO₃⁻ + 3H⁺ + 2e⁻ → HNO₂ + H₂O; for P and As, it is H₃EO₄ + 2H⁺ + 2e⁻ → H₃EO₃ + H₂O; and for Sb it is Sb₂O₅ + 4e⁻ + 10H⁺ → 2Sb³⁺ + 5H₂O.

Note the Pattern

In group 15, the stability of the +5 oxidation state decreases from P to Bi.

Note the Pattern

Because neutral covalent compounds of the trivalent group 15 elements have a lone pair of electrons on the central atom, they tend to be Lewis bases.

Reactions and Compounds of Nitrogen

Like carbon, nitrogen has four valence orbitals (one 2s and three 2p), so it can participate in at most four electron-pair bonds by using sp³ hybrid orbitals. Unlike carbon, however, nitrogen does not form long chains because of repulsive interactions between lone pairs of electrons on adjacent atoms. These interactions become important at the shorter internuclear distances encountered with the smaller, second-period elements of groups 15, 16, and 17. (For more information on internuclear distance, see Chapter 7 "The Periodic Table and Periodic Trends", Section 7.2 "Sizes of Atoms and Ions" and Chapter 8 "Ionic versus Covalent Bonding", Section 8.2 "Ionic Bonding".) Stable compounds with N–N bonds are limited to chains of no more than three N atoms, such as the azide ion (N₃⁻).

Nitrogen is the only pnicogen that normally forms multiple bonds with itself and other second-period elements, using π overlap of adjacent np orbitals. Thus the stable form of elemental nitrogen is N₂, whose N≡N bond is so strong (D_N≡N = 942 kJ/mol) compared with the N–N and N=N bonds (D_N–N = 167 kJ/mol; D_N=N = 418 kJ/mol) that all compounds containing N–N and N=N bonds are thermodynamically unstable with respect to the formation of N₂. In fact, the formation of the N≡N bond is so thermodynamically favored that virtually all compounds containing N–N bonds are potentially explosive.

Again in contrast to carbon, nitrogen undergoes only two important chemical reactions at room temperature: it reacts with metallic lithium to form lithium nitride, and it is reduced to ammonia by certain microorganisms. (For more information lithium, see Chapter 21 "Periodic Trends and the ".) At higher temperatures, however, N₂ reacts with more electropositive elements, such as those in group 13, to give binary nitrides, which range from covalent to ionic in character. Like the corresponding compounds of carbon, binary compounds of nitrogen with oxygen, hydrogen, or other nonmetals are usually covalent molecular substances.

Few binary molecular compounds of nitrogen are formed by direct reaction of the elements. At elevated temperatures, N₂ reacts with H₂ to form ammonia, with O₂ to form a mixture of NO and NO₂, and with carbon to form cyanogen (N≡C–C≡N); elemental nitrogen does not react with the halogens or the other chalcogens. Nonetheless, all the binary nitrogen halides (NX₃) are known. Except for NF₃, all are toxic, thermodynamically unstable, and potentially explosive, and all are prepared by reacting the halogen with NH₃ rather than N₂. Both nitrogen monoxide (NO) and nitrogen dioxide (NO₂) are thermodynamically unstable, with positive free energies of formation. Unlike NO, NO₂ reacts readily with excess water, forming a 1:1 mixture of nitrous acid (HNO₂) and nitric acid (HNO₃):

Equation 22.29

2NO₂(g) + H₂O(l) → HNO₂(aq) + HNO₃(aq)

Nitrogen also forms N₂O (dinitrogen monoxide, or nitrous oxide), a linear molecule that is isoelectronic with CO₂ and can be represented as ⁻N=N⁺=O. Like the other two oxides of nitrogen, nitrous oxide is thermodynamically unstable. The structures of the three common oxides of nitrogen are as follows:

Note the Pattern

Few binary molecular compounds of nitrogen are formed by the direct reaction of the elements.

At elevated temperatures, nitrogen reacts with highly electropositive metals to form ionic nitrides, such as Li₃N and Ca₃N₂. These compounds consist of ionic lattices formed by Mⁿ⁺ and N³⁻ ions. Just as boron forms interstitial borides and carbon forms interstitial carbides, with less electropositive metals nitrogen forms a range of interstitial nitrides, in which nitrogen occupies holes in a close-packed metallic structure. Like the interstitial carbides and borides, these substances are typically very hard, high-melting materials that have metallic luster and conductivity.

Nitrogen also reacts with semimetals at very high temperatures to produce covalent nitrides, such as Si₃N₄ and BN, which are solids with extended covalent network structures similar to those of graphite or diamond. Consequently, they are usually high melting and chemically inert materials.

Ammonia (NH₃) is one of the few thermodynamically stable binary compounds of nitrogen with a nonmetal. It is not flammable in air, but it burns in an O₂ atmosphere:

Equation 22.30

4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(g)

About 10% of the ammonia produced annually is used to make fibers and plastics that contain amide bonds, such as nylons and polyurethanes, while 5% is used in explosives, such as ammonium nitrate, TNT (trinitrotoluene), and nitroglycerine. Large amounts of anhydrous liquid ammonia are used as fertilizer.

Nitrogen forms two other important binary compounds with hydrogen. Hydrazoic acid (HN₃), also called hydrogen azide, is a colorless, highly toxic, and explosive substance. Hydrazine (N₂H₄) is also potentially explosive; it is used as a rocket propellant and to inhibit corrosion in boilers.

Note the Pattern

B, C, and N all react with transition metals to form interstitial compounds that are hard, high-melting materials.

Example 5

For each reaction, explain why the given products form when the reactants are heated.

Sr(s) + N₂O(g) $\overset{Δ}{\to}$ SrO(s) + N₂(g)
NH₄NO₂(s) $\overset{Δ}{\to}$ N₂(g) + 2H₂O(g)
Pb(NO₃)₂(s) $\overset{Δ}{\to}$ PbO₂(s) + 2NO₂(g)

Given: balanced chemical equations

Asked for: why the given products form

Strategy:

Classify the type of reaction. Using periodic trends in atomic properties, thermodynamics, and kinetics, explain why the observed reaction products form.

Solution:

As an alkali metal, strontium is a strong reductant. If the other reactant can act as an oxidant, then a redox reaction will occur. Nitrous oxide contains nitrogen in a low oxidation state (+1), so we would not normally consider it an oxidant. Nitrous oxide is, however, thermodynamically unstable ( $Δ H_{f}^{\circ}$ > 0 and $Δ G_{f}^{\circ}$ > 0), and it can be reduced to N₂, which is a stable species. Consequently, we predict that a redox reaction will occur.
When a substance is heated, a decomposition reaction probably will occur, which often involves the release of stable gases. In this case, ammonium nitrite contains nitrogen in two different oxidation states (−3 and +3), so an internal redox reaction is a possibility. Due to its thermodynamic stability, N₂ is the probable nitrogen-containing product, whereas we predict that H and O will combine to form H₂O.
Again, this is probably a thermal decomposition reaction. If one element is in an usually high oxidation state and another in a low oxidation state, a redox reaction will probably occur. Lead nitrate contains the Pb²⁺ cation and the nitrate anion, which contains nitrogen in its highest possible oxidation state (+5). Hence nitrogen can be reduced, and we know that lead can be oxidized to the +4 oxidation state. Consequently, it is likely that lead(II) nitrate will decompose to lead(IV) oxide and nitrogen dioxide when heated. Even though PbO₂ is a powerful oxidant, the release of a gas such as NO₂ can often drive an otherwise unfavorable reaction to completion (Le Châtelier’s principle). Note, however, that PbO₂ will probably decompose to PbO at high temperatures.

Exercise

Predict the product(s) of each reaction and write a balanced chemical equation for each reaction.

NO(g) + H₂O(l) $\overset{Δ}{\to}$
NH₄NO₃(s) $\overset{Δ}{\to}$
Sr(s) + N₂(g) →

Answer:

NO(g) + H₂O(l) $\overset{Δ}{\to}$ no reaction
NH₄NO₃(s) $\overset{Δ}{\to}$ N₂O(g) + 2H₂O(g)
3Sr(s) + N₂(g) → Sr₃N₂(s)

Reactions and Compounds of the Heavier Pnicogens

Like the heavier elements of group 14, the heavier pnicogens form catenated compounds that contain only single bonds, whose stability decreases rapidly as we go down the group. For example, phosphorus exists as multiple allotropes, the most common of which is white phosphorus, which consists of P₄ tetrahedra and behaves like a typical nonmetal. As is typical of a molecular solid, white phosphorus is volatile, has a low melting point (44.1°C), and is soluble in nonpolar solvents. It is highly strained, with bond angles of only 60°, which partially explains why it is so reactive and so easily converted to more stable allotropes. Heating white phosphorus for several days converts it to red phosphorus, a polymer that is air stable, virtually insoluble, denser than white phosphorus, and higher melting, properties that make it much safer to handle. A third allotrope of phosphorus, black phosphorus, is prepared by heating the other allotropes under high pressure; it is even less reactive, denser, and higher melting than red phosphorus. As expected from their structures, white phosphorus is an electrical insulator, and red and black phosphorus are semiconductors. The three heaviest pnicogens—arsenic, antimony, and bismuth—all have a metallic luster, but they are brittle (not ductile) and relatively poor electrical conductors.

Note the Pattern

As in group 14, the heavier group 15 elements form catenated compounds that contain only single bonds, whose stability decreases as we go down the group.

The reactivity of the heavier pnicogens decreases as we go down the column. Phosphorus is by far the most reactive of the pnicogens, forming binary compounds with every element in the periodic table except antimony, bismuth, and the noble gases. Phosphorus reacts rapidly with O₂, whereas arsenic burns in pure O₂ if ignited, and antimony and bismuth react with O₂ only when heated. None of the pnicogens reacts with nonoxidizing acids such as aqueous HCl, but all dissolve in oxidizing acids such as HNO₃. Only bismuth behaves like a metal, dissolving in HNO₃ to give the hydrated Bi³⁺ cation.

Note the Pattern

The reactivity of the heavier group 15 elements decreases as we go down the column.

The heavier pnicogens can use energetically accessible 3d, 4d, or 5d orbitals to form dsp³ or d²sp³ hybrid orbitals for bonding. Consequently, these elements often have coordination numbers of 5 or higher. Phosphorus and arsenic form halides (e.g., AsCl₅) that are generally covalent molecular species and behave like typical nonmetal halides, reacting with water to form the corresponding oxoacids (in this case, H₃AsO₄). All the pentahalides are potent Lewis acids that can expand their coordination to accommodate the lone pair of a Lewis base:

Equation 22.31

AsF₅(soln) + F⁻(soln) → AsF₆⁻(soln)

In contrast, bismuth halides have extended lattice structures and dissolve in water to produce hydrated ions, consistent with the stronger metallic character of bismuth.

Except for BiF₃, which is essentially an ionic compound, the trihalides are volatile covalent molecules with a lone pair of electrons on the central atom. Like the pentahalides, the trihalides react rapidly with water. In the cases of phosphorus and arsenic, the products are the corresponding acids, H₃PO₃ and H₃AsO₃, where E is P or As:

Equation 22.32

EX₃(l) + 3H₂O(l) → H₃EO₃(aq) + 3HX(aq)

Phosphorus halides are also used to produce insecticides, flame retardants, and plasticizers.

With energetically accessible d orbitals, phosphorus and, to a lesser extent, arsenic are able to form π bonds with second-period atoms such as N and O. This effect is even more important for phosphorus than for silicon, resulting in very strong P–O bonds and even stronger P=O bonds. The first four elements in group 15 also react with oxygen to produce the corresponding oxide in the +3 oxidation state. Of these oxides, P₄O₆ and As₄O₆ have cage structures formed by inserting an oxygen atom into each edge of the P₄ or As₄ tetrahedron (part (a) in Figure 22.11 "The Structures of Some Cage Compounds of Phosphorus"), and they behave like typical nonmetal oxides. For example, P₄O₆ reacts with water to form phosphorous acid (H₃PO₃). Consistent with its position between the nonmetal and metallic oxides, Sb₄O₆ is amphoteric, dissolving in either acid or base. In contrast, Bi₂O₃ behaves like a basic metallic oxide, dissolving in acid to give solutions that contain the hydrated Bi³⁺ ion. The two least metallic elements of the heavier pnicogens, phosphorus and arsenic, form very stable oxides with the formula E₄O₁₀ in the +5 oxidation state (part (b) in Figure 22.11 "The Structures of Some Cage Compounds of Phosphorus"). In contrast, Bi₂O₅ is so unstable that there is no absolute proof it exists.

Figure 22.11 The Structures of Some Cage Compounds of Phosphorus

(a, b) The structures of P₄O₆ and P₄O₁₀ are both derived from the structure of white phosphorus (P₄) by inserting an oxygen atom into each of the six edges of the P₄ tetrahedron; P₄O₁₀ contains an additional terminal oxygen atom bonded to each phosphorus atom. (c) The structure of P₄S₃ is also derived from the structure of P₄ by inserting three sulfur atoms into three adjacent edges of the tetrahedron.

The heavier pnicogens form sulfides that range from molecular species with three-dimensional cage structures, such as P₄S₃ (part (c) in Figure 22.11 "The Structures of Some Cage Compounds of Phosphorus"), to layered or ribbon structures, such as Sb₂S₃ and Bi₂S₃, which are semiconductors. Reacting the heavier pnicogens with metals produces substances whose properties vary with the metal content. Metal-rich phosphides (such as M₄P) are hard, high-melting, electrically conductive solids with a metallic luster, whereas phosphorus-rich phosphides (such as MP₁₅) are lower melting and less thermally stable because they contain catenated P_n units. Many organic or organometallic compounds of the heavier pnicogens containing one to five alkyl or aryl groups are also known. Because of the decreasing strength of the pnicogen–carbon bond, their thermal stability decreases from phosphorus to bismuth.

Note the Pattern

Phosphorus has the greatest ability to form π bonds with elements such as O, N, and C.

Note the Pattern

The thermal stability of organic or organometallic compounds of group 15 decreases down the group due to the decreasing strength of the pnicogen–carbon bond.

Example 6

For each reaction, explain why the given products form.

$Bi(s) + \frac{3}{2} Br(l) \to {BiBr}_{3} (s)$
2(CH₃)₃As(l) + O₂(g) → 2(CH₃)₃As=O(s)
PBr₃(l) + 3H₂O(l) → H₃PO₃(aq) + 3HBr(aq)
As(s) + Ga(s) $\overset{Δ}{\to}$ GaAs(s)

Given: balanced chemical equations

Asked for: why the given products form

Strategy:

Classify the type of reaction. Using periodic trends in atomic properties, thermodynamics, and kinetics, explain why the reaction products form.

Solution:

Bromine is an oxidant, and bismuth is a metal that can be oxidized. Hence a redox reaction is likely to occur. To identify the product, recall that bismuth can form compounds in either the +3 or +5 oxidation state. The heaviest pnicogen, bismuth is rather difficult to oxidize to the +5 oxidation state because of the inert-pair effect. Hence the product will probably be bismuth(III) bromide.
Trimethylarsine, with a lone pair of electrons on the arsenic atom, can act as either a Lewis base or a reductant. If arsenic is oxidized by two electrons, then oxygen must be reduced, most probably by two electrons to the −2 oxidation state. Because As(V) forms strong bonds to oxygen due to π bonding, the expected product is (CH₃)₃As=O.
Phosphorus tribromide is a typical nonmetal halide. We expect it to react with water to produce an oxoacid of P(III) and the corresponding hydrohalic acid.Because of the strength of the P=O bond, phosphorous acid (H₃PO₃) is actually HP(O)(OH)₂, which contains a P=O bond and a P–H bond.
Gallium is a metal with a strong tendency to act as a reductant and form compounds in the +3 oxidation state. In contrast, arsenic is a semimetal. It can act as a reductant to form compounds in the +3 or +5 oxidation state, or it can act as an oxidant, accepting electrons to form compounds in the −3 oxidation state. If a reaction occurs, then a binary compound will probably form with a 1:1 ratio of the elements. GaAs is an example of a III-V compound, many of which are used in the electronics industry. (For more information on electrical properties, see Chapter 12 "Solids".)

Exercise

Predict the products of each reaction and write a balanced chemical equation for each reaction.

PCl₅(s) + H₂O(l) →
Bi₂O₅(s) $\overset{Δ}{\to}$
Ca₃P₂(s) + H⁺(aq) →
NaNH₂(s) + PH₃(soln) →

Answer:

PCl₅(s) + 4H₂O(l) → H₃PO₄(aq) + 5HCl(aq)
Bi₂O₅(s) $\overset{Δ}{\to}$ Bi₂O₃(s) + O₂(g)
Ca₃P₂(s) + 6H⁺(aq) → 2PH₃(g) + 3Ca²⁺(aq)
NaNH₂(s) + PH₃(soln) → NaPH₂(s) + NH₃(soln)

Summary

In group 15, nitrogen and phosphorus behave chemically like nonmetals, arsenic and antimony behave like semimetals, and bismuth behaves like a metal. Nitrogen forms compounds in nine different oxidation states. The stability of the +5 oxidation state decreases from phosphorus to bismuth because of the inert-pair effect. Due to their higher electronegativity, the lighter pnicogens form compounds in the −3 oxidation state. Because of the presence of a lone pair of electrons on the pnicogen, neutral covalent compounds of the trivalent pnicogens are Lewis bases. Nitrogen does not form stable catenated compounds because of repulsions between lone pairs of electrons on adjacent atoms, but it does form multiple bonds with other second-period atoms. Nitrogen reacts with electropositive elements to produce solids that range from covalent to ionic in character. Reaction with electropositive metals produces ionic nitrides, reaction with less electropositive metals produces interstitial nitrides, and reaction with semimetals produces covalent nitrides. The reactivity of the pnicogens decreases with increasing atomic number. Compounds of the heavier pnicogens often have coordination numbers of 5 or higher and use dsp³ or d²sp³ hybrid orbitals for bonding. Because phosphorus and arsenic have energetically accessible d orbitals, these elements form π bonds with second-period atoms such as O and N. Phosphorus reacts with metals to produce phosphides. Metal-rich phosphides are hard, high-melting, electrically conductive solids with metallic luster, whereas phosphorus-rich phosphides, which contain catenated phosphorus units, are lower melting and less thermally stable.

Key Takeaway

The reactivity of the heavier group 15 elements decreases down the group, as does the stability of their catenated compounds.

Conceptual Problems

Nitrogen is the first diatomic molecule in the second period of elements. Why is N₂ the most stable form of nitrogen? Draw its Lewis electron structure. What hybrid orbitals are used to describe the bonding in this molecule? Is the molecule polar?
The polymer (SN)_n has metallic luster and conductivity. Are the constituent elements in this polymer metals or nonmetals? Why does the polymer have metallic properties?
Except for NF₃, all the halides of nitrogen are unstable. Explain why NF₃ is stable.
Which of the group 15 elements forms the most stable compounds in the +3 oxidation state? Explain why.
Phosphorus and arsenic react with the alkali metals to produce salts with the composition M₃Z₁₁. Compare these products with those produced by reaction of P and As with the alkaline earth metals. What conclusions can you draw about the types of structures favored by the heavier elements in this part of the periodic table?

Structure and Reactivity

PF₃ reacts with F₂ to produce PF₅, which in turn reacts with F⁻ to give salts that contain the PF₆⁻ ion. In contrast, NF₃ does not react with F₂, even under extreme conditions; NF₅ and the NF₆⁻ ion do not exist. Why?
Red phosphorus is safer to handle than white phosphorus, reflecting their dissimilar properties. Given their structures, how do you expect them to compare with regard to reactivity, solubility, density, and melting point?
Bismuth oxalate [Bi₂(C₂O₄)₃] is a poison. Draw its structure and then predict its solubility in H₂O, dilute HCl, and dilute HNO₃. Predict its combustion products. Suggest a method to prepare bismuth oxalate from bismuth.
Small quantities of NO can be obtained in the laboratory by reaction of the iodide ion with acidic solutions of nitrite. Write a balanced chemical equation that represents this reaction.
Although pure nitrous acid is unstable, dilute solutions in water are prepared by adding nitrite salts to aqueous acid. Write a balanced chemical equation that represents this type of reaction.
Metallic versus nonmetallic behavior becomes apparent in reactions of the elements with an oxidizing acid, such as HNO₃. Write balanced chemical equations for the reaction of each element of group 15 with nitric acid. Based on the products, predict which of these elements, if any, are metals and which, if any, are nonmetals.
Predict the product(s) of each reaction and then balance each chemical equation.
1. P₄O₁₀(s) + H₂O(l) →
2. AsCl₃(l) + H₂O(l) →
3. Bi₂O₃(s) + H₂O(l) →
4. Sb₄O₆(s) + OH⁻(aq) →
5. (C₂H₅)₃Sb(l) + O₂(g) $\overset{Δ}{\to}$
6. SbCl₃(s) + LiAlH₄(soln) →
7. Ca(s) + N₂O(g) $\overset{Δ}{\to}$
Write a balanced chemical equation to show how you would prepare each compound.
1. H₃PO₄ from P
2. Sb₂O₅ from Sb
3. SbH₃ from Sb

Answers

NaNO₂(s) + HCl(aq) → HNO₂(aq) + NaCl(aq)
1. P₄O₁₀(s) + 6H₂O(l) → 4H₃PO₄(aq)
2. AsCl₃(l) + 3H₂O(l) → H₃AsO₃(aq) + 3HCl(aq)
3. Bi₂O₃(s) + 3H₂O(l) → 2Bi(OH)₃(s)
4. Sb₄O₆(s) + 4OH⁻(aq) + 2H₂O(l) → 4H₂SbO₃⁻(aq)
5. 2(C₂H₅)₃Sb(l) + 21O₂(g) $\overset{Δ}{\to}$ 12CO₂(g) + 15H₂O(g) + Sb₂O₃(s)
6. 4SbCl₃(s) + 3LiAlH₄(soln) → 4SbH₃(g) + 3LiCl(soln) + 3AlCl₃(soln)
7. Ca(s) + N₂O(g) $\overset{Δ}{\to}$ CaO(s) + N₂(g)