How do you know h2o is a molecule?

Answer 1

because it's composed of more than a single atom.

H2O has three atoms(2 hydrogen, 1 oxygen), so it's a molecule

H has one atom, so it's an atom

H2 has two atoms, so it's a molecule

Answer 2

H2O is a molecule because it is made up of chemically bonded atoms of hydrogen and oxygen. Molecules are formed when two or more atoms bond together through interactions of their electrons to create a stable structure, as is the case with H2O.