As you have read in the previous post, when two atoms of different elements are joined together in covalent bonding, the sharing of electrons is not always equal, creating permanent partial charges on each atom. These types of molecules are known as polar molecules.
However, this is insufficient for determining if polyatomic molecules (E.g. CO2, H2O etc) are polar in nature. To determine if these molecules are polar, what matters most is the net dipole moment.
Consider CO2, which has a linear molecular shape. Because oxygen is more electronegative than carbon (check by calculating the ENC of oxygen and carbon), this gives rise to 2 dipole moments pointing in opposite directions. Hence, the dipole moments cancel each other and CO2 is said to be non-polar.
However, water has a bent molecular shape. Oxygen is more electronegative than hydrogen. Thus this gives rise to 2 dipole moments as seen below.
But, in this case, water is a polar molecule because of its bent structure. The horizontal components of both dipole moments are in the opposite direction and hence will cancel out each other. But, the vertical components of the dipole moments are pointing in the same direction, giving rise to a net dipole moment as seen in the diagram below. Thus water is said to be a polar molecule.
Recall that the dipole moment points from the partial positive end to the partial negative end. Thus water is a polar molecule with a permanent partial negative charge (δ – ) on the oxygen atom and permanent partial positive charges (δ + ) on the hydrogen atoms.
*Note: I have not addressed the issue of why CO2 is linear but H2O is bent. This has to do with the Valence Shell Electron Pair Repulsion (VSEPR) theory which is too long to write about in a short blog post. Furthermore, it will be covered in great detail by your JC teachers. For those of you who are really interested or have too much free time, you can read about it here.