Molecular formula of a compound tells us how the atoms are bonded with each other in that particular molecule.
And Atomicity not atomic city.
Atomicity tell us that how many atoms are in one molecule of a particular substance.
therefore: molecular formula of water is H2O i.e., two atoms of Hydrogen are bonded with a single atom of Oxygen.
It's atomicity: 3 as there are total 3 atoms in a water molecule.