In addition:

Ca₃P₂ + 6HCl = 2РН₃ + 3 CaCl₂

Properties. Let us compare its physical properties with NH₃:

PH₃ molecule can be treated as an NH₃ analogue. Difference in the properties of PH₃ and NH₃ is associated with the following peculiarities of their structure:

• the length of bond N—H (0.1014 nm) is much shorter than the length of P—H bond (0.142 nm)

• NH₃ polarity (dipole moment is 1.47 D) significantly exceeds the polarity of PH₃ (0.55 D).

This is attributed to the absence of sp³-hybridization of valence orbitals in PH₃, which is typical of NH₃. Indeed, PH₃ molecule is pyramidal,  H-P-H = 93.5^o ( 90^o) (in NH₃ – 107.3^o). It indicates the decrease of contribution of s-orbital in the hybrid bonds of PH₃ formation, i.e. unshared electron pair of P is s-orbital (in case of NH₃ at the sp³-orbitals).

This PH₃ structure considerably complicates formation of donor-acceptor bonds with electron donors. Therefore, PH₃ has very weak basic properties.

The bond p—h is less strong (322 kJ / mol) than n—n (390 kJ / mol), ph3 practically does not form h-bonds. As, Sb, Bi Compounds e (-3)

The oxidation state –3 of these elements is found in arsenides, antimonides bismutides of s-elements of I and II groups. Metal-like compounds are formed in most of other cases (s-elements form salt-like compounds).

In the series of nitrides, phosphides, arsenides, antimonides, and bismutides of the same type gap width decreases, indicating an increase of contribution of delocalised bond (unlike covalent bonding that is localised along the bond axis of molecular structure compounds):

AlN AlP AlAs AlSb

 E, eV 3.8 3.0 2.16 1.6

Most of these compounds decompose under the action of acids.

Hydrogen compounds.

As, Sb, Bi do not interact with hydrogen, so EH₃ is available only indirectly:

Mg₃Е₂ + 6HCl = 3MgCl₂ + 2H₃Е

or under the action of Zn on compounds of arsenic in acidic medium:

As₂O₃ + 6Zn + 12HCl = 2AsH₃ + 6ZnCl₂ + 3H₂O

This is known as the Marsh reaction (test for arcenic).

Arsine, AsH₃, stibine, SbH₃, bismuthine, BiH₃, are gases with a sharp smell. They, especially AsH₃, are one of the strongest inorganic poisons (generally, all volatile compounds and soluble compounds of arsenic are extremely poisonous). These compounds are also strongly endothermic, and therefore very unstable. Generally, in the series AsH₃—SbH₃—BiH₃ stability decreases. BiH₃ decomposes at the moment of liberation and its properties are not well studied.

	dE-H, nm	ЕЕ-Н, kJ/mol	Ноf, kJ/mol	НЕН	 ЕН3, D
NH3	0.101	390.4	-46.4	107.3o (sp3)	1.47
PH3	0.142	328.5	5.4	93.3 o ( - )	0.58
AsH3	0.152	279.2	66.5	92 o	0.22
SbH3	0.171	254.6	145	91 o	0.12
BiH3			230 (evaluation)

In the series of nonmetallic elements As—Sb—Bi is revealed the nonmetallicity weakening of EH₃ compounds. The structure of these molecules is similar to NH₃ and PH₃, (i.e., pyramidal structure with the atom of an element at the top and hydrogen atoms below). However, R_a is increased in the series N—P—As ... Bi and E-H polarity, the strength of bond is reduced; nonbonding orbital occupied by two electrons becomes less directed and the valence angle  HEH is decreased either.

The important property of EH₃ is their high reducing ability. For instance, arsine, AsH₃, can even reduce phosphorous acid H₃PO₃ to hypophosphorous H₃PO₂, although phosphorous acid itself is a very active reducing agent:

2 AsH₃ + 3 Н₃РО₃ = 3 Н₃РО₂ + 2 As + 3 Н₂О ( G^o = -60 kJ / mol)

EН₃ decomposes easily: 2EН₃ = 2E + 3H₂O

All hydrides EН₃ are dissolved well in water, but do not interact chemically. They burn in the air:

2EН₃ + 3О₂ = E₂О₃ + 3Н₂О

In the series NH₃—PH₃—AsH₃—SbH₃—BiH₃ donor properties of molecules are weakened, so the typical of NH₃ synthesis reaction is not observed in case of AsH₃ and SbH₃.

Properties of hydrides can be generalized as follows: