The reaction can be unimolecular when one reacting species is involved,
for example, decomposition of ammonium nitrite Rationalised 2023-24
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Chemistry
NH4NO2 ® N2 + 2H2O
Bimolecular reactions involve simultaneous collision between two
species, for example, dissociation of hydrogen iodide 2HI ® H2 + I2
Trimolecular or termolecular reactions involve simultaneous collision
between three reacting species, for example,
2NO + O2 ® 2NO2
The probability that more than three molecules can collide and
react simultaneously is very small Hence, reactions with the
molecularity three are very rare and slow to proceed