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It is polar because of the electronegativity difference between nitrogen and oxygen. As you may know, electronegativity is the tendency of an atom to attract electrons. Because oxygen has a higher electronegativity than nitrogen, and because the difference is large enough, electrons are closer to the oxygen atoms than to the nitrogen atom. Thus, electrons are unequally shared (the definition of polar covalent bonds).
Periodic trends show that moving from left to right on the periodic table increases electronegativity. As a result, oxygen is more electronegative than nitrogen: there will be a disproportionate sharing of electrons among the oxygen atoms in the molecule making nitrogen slightly positive and the oxygens slightly negative.
It's polarity is due to the difference in electronegativites between nitrogen and oxygen, but also because its molecular shape. Due to a lone pair on Nitrogen, the central atom, it causes the shape to be trigonal planar, therefore the dipole moments between nitrogen and oxygen do not cancel out, causing an overall polar molecule.
The dipole moments do not cancel out in NO2 similarly to SO2, but unlike CO2 because CO2 has a linear shape. NO2 and SO2 both have lone pairs, which due to electron repulsion cause the shape to be bent/angular, therefore the dipole vectors do not completely cancel out and you will have a polar molecule.
This is because of lone pair electrons whose repulsions cause a bent structure. This causes the charges to be unequal throughout the molecule. If the charges had equal distribution, then it would be a nonpolar molecule. However, since it is not equally distributed it would be a polar molecule.
Due to differences in electronegativity between N and O, the bonds in NO2 are polar. However, if NO2 did not have a lone pair, then these polar bonds would cancel each other out in a linear structure. With the lone pair, the bonds are not directly opposite of one another, meaning the dipole moments do in fact point in one direction, making the entire molecule polar.
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