Pnictogen

The IUPAC has called it Group 15 since 1988. Before that, in America it was called Group VA, owing to a text by H. C. Deming and the Sargent-Welch Scientific Company, while in Europe it was called Group VB, which the IUPAC had recommended in 1970. (Pronounced "group five A" and "group five B"; "V" is the Roman numeral 5.) In semiconductor physics, it is still usually called Group V. The "five" ("V") in the historical names comes from the "pentavalency" of nitrogen, reflected by the stoichiometry of compounds such as N₂O₅. They have also been called the pentels.

Characteristics

Chemical

Like other groups, the members of this family manifest similar patterns in electron configuration, notably in their valence shells, resulting in trends in chemical behavior.

This group has the defining characteristic whereby each component element has 5 electrons in their valence shell, that is, 2 electrons in the s sub-shell and 3 unpaired electrons in the p sub-shell. They are therefore 3 electrons shy of filling their valence shell in their non-ionized state. The Russell-Saunders term symbol of the ground state in all elements in the group is ⁴S_3⁄2.

The most important elements of this group to life on Earth are nitrogen (N), which in its diatomic form is the principal component of air, and phosphorus (P), which, like nitrogen, is essential to all known forms of life.

Compounds

Binary compounds of the group can be referred to collectively as pnictides. Magnetic properties of pnictide compounds span the cases of diamagnetic systems (such as BN or GaN) and magnetically ordered systems (MnSb is paramagnetic at elevated temperatures and ferromagnetic at room temperature); the former compounds are usually transparent and the latter metallic. Other pnictides include the ternary rare-earth (RE) main-group variety of pnictides. These are in the form of RE_aM_bPn_c, where M is a carbon group or boron group element and Pn is any pnictogen except nitrogen. These compounds are between ionic and covalent compounds and thus have unusual bonding properties.

These elements are also noted for their stability in compounds due to their tendency to form covalent double bonds and triple bonds. This property of these elements leads to their potential toxicity, most evident in phosphorus, arsenic, and antimony. When these substances react with various chemicals of the body, they create strong free radicals that are not easily processed by the liver, where they accumulate. Paradoxically, this same strong bonding causes nitrogen's and bismuth's reduced toxicity (when in molecules), because these strong bonds with other atoms are difficult to split, creating very unreactive molecules. For example, N₂, the diatomic form of nitrogen, is used as an inert gas in situations where using argon or another noble gas would be too expensive.

Formation of multiple bonds is facilitated by their five valence electrons whereas the octet rule permits a pnictogen for accepting three electrons on covalent bonding. Because 5 > 3, it leaves unused two electrons in a lone pair unless there is a positive charge around (like in [NH₄]⁺). When a pnictogen forms only three single bonds, effects of the lone pair typically result in trigonal pyramidal molecular geometry.

Oxidation states

The light pnictogens (nitrogen, phosphorus, and arsenic) tend to form −3 charges when reduced, completing their octet. When oxidized or ionized, pnictogens typically take an oxidation state of +3 (by losing all three p-shell electrons in the valence shell) or +5 (by losing all three p-shell and both s-shell electrons in the valence shell). However heavier pnictogens are more likely to form the +3 oxidation state than lighter ones due to the s-shell electrons becoming more stabilized.

−3 oxidation state

Pnictogens can react with hydrogen to form pnictogen hydrides such as ammonia. Going down the group, to phosphane (phosphine), arsane (arsine), stibane (stibine), and finally bismuthane (bismuthine), each pnictogen hydride becomes progressively less stable (more unstable), more toxic, and has a smaller hydrogen-hydrogen angle (from 107.8° in ammonia to 90.48° in bismuthane). (Also, technically, only ammonia and phosphane have the pnictogen in the −3 oxidation state because, for the rest, the pnictogen is less electronegative than hydrogen.)

Crystal solids featuring pnictogens fully reduced include yttrium nitride, calcium phosphide, sodium arsenide, indium antimonide, and even double salts like aluminum gallium indium phosphide. These include III-V semiconductors, including gallium arsenide, the second-most widely used semiconductor after silicon.

+3 oxidation state

Nitrogen forms a limited number of stable III compounds. Nitrogen(III) oxide can only be isolated at low temperatures, and nitrous acid is unstable. Nitrogen trifluoride is the only stable nitrogen trihalide, with nitrogen trichloride, nitrogen tribromide, and nitrogen triiodide being explosive—nitrogen triiodide being so shock-sensitive that the touch of a feather detonates it (the last three actually feature nitrogen in the -3 oxidation state). Phosphorus forms a +III oxide which is stable at room temperature, phosphorous acid, and several trihalides, although the triiodide is unstable. Arsenic forms +III compounds with oxygen as arsenites, arsenous acid, and arsenic(III) oxide, and it forms all four trihalides. Antimony forms antimony(III) oxide and antimonite but not oxyacids. Its trihalides, antimony trifluoride, antimony trichloride, antimony tribromide, and antimony triiodide, like all pnictogen trihalides, each have trigonal pyramidal molecular geometry.

The +3 oxidation state is bismuth's most common oxidation state because its ability to form the +5 oxidation state is hindered by relativistic properties on heavier elements, effects that are even more pronounced concerning moscovium. Bismuth(III) forms an oxide, an oxychloride, an oxynitrate, and a sulfide. Moscovium(III) is predicted to behave similarly to bismuth(III). Moscovium is predicted to form all four trihalides, of which all but the trifluoride are predicted to be soluble in water. It is also predicted to form an oxychloride and oxybromide in the +III oxidation state.

+5 oxidation state

For nitrogen, the +5 state is typically serves as only a formal explanation of molecules like N₂O₅, as the high electronegativity of nitrogen causes the electrons to be shared almost evenly. Pnictogen compounds with coordination number 5 are hypervalent. Nitrogen(V) fluoride is only theoretical and has not been synthesized. The "true" +5 state is more common for the essentially non-relativistic typical pnictogens phosphorus, arsenic, and antimony, as shown in their oxides, phosphorus(V) oxide, arsenic(V) oxide, and antimony(V) oxide, and their fluorides, phosphorus(V) fluoride, arsenic(V) fluoride, antimony(V) fluoride. They also form related fluoride-anions, hexafluorophosphate, hexafluoroarsenate, hexafluoroantimonate, that function as non-coordinating anions. Phosphorus even forms mixed oxide-halides, known as oxyhalides, like phosphorus oxychloride, and mixed pentahalides, like phosphorus trifluorodichloride. Pentamethylpnictogen(V) compounds exist for arsenic, antimony, and bismuth. However, for bismuth, the +5 oxidation state becomes rare due to the relativistic stabilization of the 6s orbitals known as the inert-pair effect, so that the 6s electrons are reluctant to bond chemically. This causes bismuth(V) oxide to be unstable and bismuth(V) fluoride to be more reactive than the other pnictogen pentafluorides, making it an extremely powerful fluorinating agent. This effect is even more pronounced for moscovium, prohibiting it from attaining a +5 oxidation state.

Other oxidation states

• Nitrogen forms a variety of compounds with oxygen in which the nitrogen can take on a variety of oxidation states, including +II, +IV, and even some mixed-valence compounds and very unstable +VI oxidation state.
• In hydrazine, diphosphane, and organic derivatives of the two, the nitrogen or phosphorus atoms have the −2 oxidation state. Likewise, diimide, which has two nitrogen atoms double-bonded to each other, and its organic derivatives have nitrogen in the oxidation state of −1.
  • Similarly, realgar has arsenic–arsenic bonds, so the arsenic's oxidation state is +II.
  • A corresponding compound for antimony is Sb₂(C₆H₅)₄, where the antimony's oxidation state is +II.
• Phosphorus has the +1 oxidation state in hypophosphorous acid and the +4 oxidation state in hypophosphoric acid.
• Antimony tetroxide is a mixed-valence compound, where half of the antimony atoms are in the +3 oxidation state, and the other half are in the +5 oxidation state.
• It is expected that moscovium will have an inert-pair effect for both the 7s and the 7p_1/2 electrons, as the binding energy of the lone 7p_3/2 electron is noticeably lower than that of the 7p_1/2 electrons. This is predicted to cause +I to be a common oxidation state for moscovium, although it also occurs to a lesser extent for bismuth and nitrogen.

Physical

The pnictogens exemplify the transition from nonmetal to metal going down the periodic table: a gaseous diatomic nonmetal (N), two elements displaying many allotropes of varying conductivities and structures (P and As), and then at least two elements that only form metallic structures in bulk (Sb and Bi; probably Mc as well). All the elements in the group are solids at room temperature, except for nitrogen which is gaseous at room temperature. Nitrogen and bismuth, despite both being pnictogens, are very different in their physical properties. For instance, at STP nitrogen is a transparent non-metallic gas, while bismuth is a silvery-white metal.

The densities of the pnictogens increase towards the heavier pnictogens. Nitrogen's density is 0.001251 g/cm³ at STP. Phosphorus's density is 1.82 g/cm³ at STP, arsenic's is 5.72 g/cm³, antimony's is 6.68 g/cm³, and bismuth's is 9.79 g/cm³.

Nitrogen's melting point is −210 °C and its boiling point is −196 °C. Phosphorus has a melting point of 44 °C and a boiling point of 280 °C. Arsenic is one of only two elements to sublimate at standard pressure; it does this at 603 °C. Antimony's melting point is 631 °C and its boiling point is 1587 °C. Bismuth's melting point is 271 °C and its boiling point is 1564 °C.

Nitrogen's crystal structure is hexagonal. Phosphorus's crystal structure is cubic. Arsenic, antimony, and bismuth all have rhombohedral crystal structures.

Nuclear

All pnictogens up to antimony have at least one stable isotope; bismuth has no stable isotopes, but has a primordial radioisotope with a half-life much longer than the age of the universe (²⁰⁹Bi); and all known isotopes of moscovium are synthetic and highly radioactive. In addition to these isotopes, traces of ¹³N, ³²P, and ³³P occur in nature, along with various bismuth isotopes (other than ²⁰⁹Bi) in the decay chains of thorium and uranium.