Question

how to find the atomicity of H2O

Answer 1

There is a simple way to determine the atomicity of any element. It can be determined as a ratio of molecular mass and atomic mass. For example, using thecase of oxygen molecule, the molecular mass is 32 (= 2x16), while the atomic mass is 16. If we divide the two, we get atomicity as 2 (=32/16).

Answer 2

here is your answer
two atoms of hydrogen and one atom of Oxygen is present
therfore atomicity is 3