The partial negative charge in a molecule of water occurs because the electrons shared between the oxygen and hydrogen stoms spend more time around the oxygen atom nucleus than around the hydrogen atom nucleus?
Partial charge is due to difference in electron negativity between two atoms in a molecule. Electrons are attracted towards the atoms with more electro-negativity. In water molecule oxygen has more electro-negativity as compared to oxygen so it is more attracted towards oxygen and thus there is partial charge on it.
