Q.NH3 forms hydrogen bonds but PH3 does not - Why?

Acidic strength of group-15 pentoxides decreases down the group; N2O5 (top of the group) is the most acidic.

All the oxides listed are +5 oxides of group-15 elements: N2O5, P2O5, As2O5, Sb2O5. Going down the group the elements become larger and more metallic, so their oxides become progressively less acidic (more basic/amphoteric):

N2O5 > P2O5 > As2O5 > Sb2O5 (decreasing acidic strength)

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Basicity of group-15 hydrides decreases down the group, so NH3 is the strongest base.

In the hydrides of group 15 (NH3, PH3, AsH3, SbH3), basicity depends on the availability of the lone pair on the central atom for donation. As we go down the group the atomic size increases, the lone pair occupies a larger, more diffuse orbital, and its electron density (and hence donor ability) falls. Therefore basic strength dec …

Nitrogen lacks d orbitals in the second shell, so its maximum covalency is 4.

Nitrogen is a second-period element; its valence shell (n = 2) has only 2s and 2p orbitals and no d orbitals. It can therefore form a maximum of four covalent bonds by using its four available orbitals, as in the ammonium ion NH4+ (three normal N-H bonds plus one coordinate bond). It cannot expand its …

Count the electrons: 2 + 2 + 1 + 1 + 1 = 7. The element with atomic number 7 is nitrogen.

Add up the electrons in 1s2 2s2 2px1 2py1 2pz1 = 2 + 2 + 1 + 1 + 1 = 7 electrons.

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Reducing character of group 15 hydrides increases down the group, so NH3 is the weakest reducing agent.

The hydrides of group 15 elements are NH3, PH3, AsH3, SbH3 and BiH3. Reducing character of a hydride depends on the strength of the E-H bond (E = group 15 element) - the weaker the E-H bond, the more easily it is broken to release H, and the stronger the reducing power.

Down the group, atomic size increases and the E-H bond becomes progressively longer and weaker, so bond dissociation enthalpy decreases from NH3 to BiH3. Hence reducing character increases in the order: …

Basicity of group 15 hydrides decreases down the group.

Among the group 15 hydrides, basic strength decreases down the group: $NH_3 > PH_3 > AsH_3 > BiH_3$. This is because as the central atom gets larger and less electronegative down the group, its lone pair becomes more diffuse and less av …

Nitrogen and oxygen are non-metals.

Nitrogen (group 15) and oxygen (group 16) lie in the upper-right p-block of the periodic table. They are electronegative, tend to gain electrons, exist as gaseous diatomic molecules (N2, O2), and do not show metall …

Inert-pair effect → Bi is most stable as +3.

In group 15 the stability of the +5 state falls and that of +3 rises down the group (inert-pair effect on the 6s² electrons). Bismuth, the heaviest, is therefore most stabl …