Here's why:
* Polar covalent bonds occur when the electrons are shared unequally. This happens because one atom has a higher electronegativity (a stronger attraction for electrons) than the other. This leads to a partial positive charge on the less electronegative atom and a partial negative charge on the more electronegative atom.
* Nonpolar covalent bonds occur when the electrons are shared equally. This happens when the two atoms have the same or very similar electronegativity.
In summary:
* Equal sharing of electrons = nonpolar covalent bond
* Unequal sharing of electrons = polar covalent bond
