Question

how NH4 is formed if nitrogen is having valency3.

Answer 1

N in NH3 has a lone pair of electrons. These two electron occupy a molecular orbital, which is not involved in bonding and is not part of the molecular orbitals which make up the three NH bonds. Note that this is not an extra electron since N already had it, before bonding. The electronic config is 1s2 2s2 2p3. 3 H atoms bond with N by sharing the 3 p orbital electrons and their own s orbital electrons.

Answer 2

Under 300 atm. pressure , Iron as catalyst and Molybdenum as promoter , Nitrogen​ and Hydrogen combine to form NH4
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