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→ All the elements of this group form trihydrides having formula MH3 where, M = N, P, As, Sb, Bi
These hydrides are as follows

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All the important properties of hydrides of group -15 are summarised in table 7.4.

Table 7.4: Properties of Hydrides of group – 15

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→ From the table it is clear that hydrides show variable change in their properties. The discussion of the properties of hydrides of group 15 is as follow:

→ Thermal Stability : Thermal stability of the hydrides of group – 15 elements decreases down the group. Because on moving down the group the size of central atom increases as a result the strength of E-H bond decreases and hence thermal stability decreases.

NH3 PH3 > AsH3 > SbH3 > BiH3
(Thermal stability)

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→ Reducing Character : The reducing character of the hydrides of group – 15 elements increases from NH, to BiHg. Because the reducing power depends on thermal stability, lesser the thermal stability more will be the reducing power. Hence NH, which has maximum thermal stability, is the weakest reducing agent among the hydrides of group – 15 elements, while BiH, is the strongest reducing agent to the least thermal stability.
NH3 < PH3 < AsH3 < SbH3 < BiH3
(Reducing power)

→ Basic Character : Hydrides of nitrogen family are Lewis base due to the presence of lone pair of electron on the central atom. The basic strength of these hydrides decreases down the group

→ NH3 PH3 > AsH3 > SbH3 > BiH3
(Basic strength)

→ In group – 15 nitrogen atom has the smallest size and a very high electron density hence it has maximum tendency to release electron. Thus have the highest basic strength. On moving down the group atomic size increases and hence the electron density on central atom decreases consequently the electron releasing tendency decreases thus basic strength decreases.

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→ Hydrogen Bond: Nitrogen due to small size and high electronegativity forms hydrogen bonds. But the other elements of this group cannot form hydrogen bonds.

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→ Melting and Boiling Points : On moving down the group melting and boiling points of the hydride of group – 15 elements because as molecular weight increases, the extent of van der Waal’s interactions increases, hence boiling and melting points increase but the melting and boiling point of ammonia (NH3) are higher than the melting and boiling point of the hydrides of other elements. It is due to the presence of H-bonding in ammonia.

PH3 < AsH3 < NH3 < SbH3 < BiH3
(Boiling Points)

PH3 < AsH3 < SbH3 NH3 < BiH3
(Melting Points)

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→ Bond Angle : Hydrides of group – 15 elements have pyramidal structure and sp hybridisation on the central atom. But due to presence of lone pair of electron the bond angle decreases. It has been found to be less then 109°28’.

On moving down the group, the bond angle further decreases as

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→ As we know that in hydrides the central atom is sp -hybridised and it is surrounded by four electron pairs i.e, three hond pair and one lone pair. Due to bond pair-lone pair repulsion, bond angle decreases. Further on, moving down the group the size of atom increases and electronegativity decreases.

→ As a result the electron density around the central atom also decreases and consequently, the repulsive interactions between the electron pairs also decreases thereby the bond angle decreases.

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→ Solubility : Ammonia is soluble in water due to the formation of inter-molecular hydrogen bonding. While the other hydrides are insoluble in water due to the absence of hydrogen bonding.

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