Question

I have a doubt
Why it's not H2O2? ​

Answer 1

because we know that valency of hydrogen is only 1 and Valency of oxygen is 2.

so by crisscross method the oxygens valency i.e 2 gets to the subscript of hydrogen and vice versa

hence it is H2O

Answer 2

it is because of the chemical reaction

2 molecule of hydrogen + oxygen is equal to H2O