Vapour density = relative atomic mass = 35.5
Molecular weight = 2 × Vapour Density
= 2 × 35.5 = 71 g
Atomicity is the number of atoms present in one molecule of that element.
Number of atoms = Mol wt\over At. wt
= 71\over 35.5
= 2
Hence, atomicity of a gas X is 2