1,2- and 1,3-Dichlorobenzenes are polar molecules while 1,4-dichlorobenzene is not polar molecule since 1,4-isomer is symmetric.

Introduction:
1,4-dichlorobenzene is a colorless solid that is commonly used as a moth repellent. It is an organic compound that contains two chlorine atoms and a benzene ring.

Polarity of 1,4-dichlorobenzene:
1,4-dichlorobenzene is a nonpolar molecule. A molecule is considered polar if the electronegativity difference between the atoms in the molecule is greater than 0.5. The electronegativity difference between chlorine and carbon in 1,4-dichlorobenzene is 0.3, which is less than 0.5. Therefore, the molecule is nonpolar.

Reason for nonpolarity:
The reason for the nonpolarity of 1,4-dichlorobenzene is due to the symmetrical arrangement of the chlorine atoms around the benzene ring. The molecule has a symmetrical tetrahedral shape which cancels out the polarity of the individual bonds between carbon and chlorine atoms. This means that the molecule has no overall dipole moment which makes it nonpolar.

Conclusion:
In conclusion, 1,4-dichlorobenzene is a nonpolar molecule due to its symmetrical tetrahedral structure which cancels out the polarity of individual bonds.