Why has water a polar covalent bond?

Answer 1

"Polar-covalent" is how chemists say that the bond in question is not massively polar, ie the bond isn't easily broken when placed in a polar solvent, but nor is the bond completely covalent (more or less equal sharing of bonding electrons between involved atoms). In short, a polar-covalent bond has electrons which spend more time near one atom than another, but not so much that the bond is polar or ionic.

Now for the why: Electronegativity. The difference in electronegativities (EN) between two atoms is a huge factor in determining where the bonding electrons like to spend their time (recall, EN is a measure of "how badly" an atom wants to hold onto electrons when bonded or in a molecule). Oxygen has an EN of about 3.5 and hydrogen about 2.2. Just from this you can see that the oxygen atom is wants electrons more than hydrogen does. The difference here is 1.3. As a general rule of thumb, if the difference in EN's in a particular molecular bond is greater than about 1.5 or 2, the compound is probably ionic. eg for NaCl the difference in EN's is 2.23 (ionic), and for CH4 it's 0.35 (covalent).

Answer 2

Water has a polar covalent bond because oxygen is more electronegative than hydrogen, causing unequal sharing of electrons in the O-H bond. This results in a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms, creating a polarity in the water molecule.