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. 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 {{chem2|RE_{
a}M_{
b}Pn_{
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, , 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 ammonium|). 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 7p1/2 electrons, as the
binding energy of the lone 7p3/2 electron is noticeably lower than that of the 7p1/2 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. 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. ==History==