Explanation of why cis-1,2-dichloroethene has no dipole moment:

Introduction:
Cis-1,2-dichloroethene is an organic compound with the chemical formula C2H2Cl2. It consists of two chlorine atoms attached to a central ethene (C2H4) molecule. The arrangement of these chlorine atoms and their bond dipoles determines whether the compound has a net dipole moment or not.

Cancellation of bond polarity:
In cis-1,2-dichloroethene, the two chlorine atoms are attached to adjacent carbon atoms on the same side of the molecule. This arrangement results in two C-Cl bonds that have bond dipoles pointing in the same direction. However, the bond dipoles can cancel each other out due to the symmetrical nature of the molecule.

Symmetrical geometry:
The carbon-carbon double bond in ethene causes the molecule to have a planar structure. The two chlorine atoms are located on the same plane, and their bond dipoles are oriented in opposite directions. This symmetrical arrangement leads to the cancellation of the dipole moments, resulting in a net dipole moment of zero.

Electronegativity difference:
The cancellation of dipole moments is possible because the chlorine atoms in cis-1,2-dichloroethene have similar electronegativity values. Electronegativity is a measure of an atom's ability to attract electrons in a chemical bond. When two atoms have similar electronegativity values, the bond between them is considered nonpolar, meaning the electron density is equally shared.

Equal and opposite dipole moments:
The C-Cl bonds in cis-1,2-dichloroethene have equal and opposite dipole moments due to the symmetrical arrangement of the molecule. The dipole moment is a vector quantity that has both magnitude and direction. In this case, the magnitudes of the bond dipoles are equal, but their directions are opposite, resulting in a net dipole moment of zero.

Conclusion:
In summary, cis-1,2-dichloroethene has no dipole moment due to the cancellation of the dipole moments of its two C-Cl bonds. The symmetrical arrangement of the molecule and the similar electronegativity values of the chlorine atoms allow for the equal and opposite dipole moments, resulting in a net dipole moment of zero.

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