Question

Why does the oxygen atom in a water molecule have a negative charge

Answer 1

In water molecule the electrons between the oxygen and the hydrogen are shared, but the sharing is not equal. Oxygen pulls electrons slightly towards itself. Therefore, the electrons are more near the oxygen, making that part of molecule(oxygen part) slightly negative. Since the electrons are not near the hydrogen so, that part of the molecule is slightly positive.

Answer 2

In the covalent bond between oxygen and hydrogen, the oxygen atom attracts electrons a bit more strongly than the hydrogen atoms. The unequal sharing of electrons gives the water molecule a slight negative charge near its oxygen atom and a slight positive charge near its hydrogen atoms.