Polarity arises from a difference in electronegativity. Oxygen is more electronegative than nitrogen and pulls the electrons from the bonds closer. They are unevenly distributed. Therefore, a N-O bond is polar with the oxygen negative and the nitrogen positive.
If the NO2 molecule were linear, the polar bonds would oppose each other and the molecule would be non-polar. Instead, a non-bonding pair of electrons are on nitrogen. The pair of electrons repel the bonds and distort the molecule into a bent geometry. The polar bonds do not negate each other, making the molecule polar.Learn more about Atoms & Molecules