Question

how to find which element is pyatomi diatomic tetra atomic etc.??​

Answer 1

see the subscript given below

such as-h2o it has2 atoms of hydrogen and 1 of oxygen so it has atomicity 1+2=3

the number of atoms present in an element is called the atomicity of an element ...

Here some examples are Monoatomic molecules Gold, Helium, and Neon. While Diatomic molecules Hydrogen, Oxygen and Hydrogen chloride. 

 i hope it helps...