How do covalent bonds differ from hydrogen bonds?
Water is held together with covalent bonds. In covalent bonds, electrons are shared between the atoms. In water, oxygen is bonded to two hydrogen atoms.
The electrons are not shared equally between the oxygen and hydrogen atoms. Because of the stronger electronegativity of oxygen, electrons are pulled more closely to the oxygen nucleus than the hydrogen nucleus which results in a partial negative charge for oxygen and a partial positive charge for hydrogen,
Due to these partial charges, the hydrogen is also attracted to the oxygen atom of a second water molecule. Similarly, the oxygen atom is attracted to the hydrogen atom of a second water molecule.
These attractions are between molecules resulting in hydrogen bonding.