Chemistry - Complete P-block Question with Solution | TestHub

The reaction of white phosphorus (P₄) with caustic soda (NaOH) in water produces phosphine (PH₃) gas and sodium hypophosphite (NaH₂PO₂). The balanced chemical equation is:

$$P_4+3NaOH+3H_{2}O⟶P{H}_3+3Na{H}_{2}P{O}_2$$

The gas evolved is phosphine (PH₃). Atomicity refers to the number of atoms in a molecule. In PH₃, there is one phosphorus atom and three hydrogen atoms.

Therefore, the atomicity of phosphine (PH₃) is $1+3=4$.

The final answer is $\boxed{4}$.