Question

Which of the following molecule /molecules will have zero dipole moment?
${H_2}O,\;C{O_2},\;CC{l_4},\;CHC{l_3},\;N{H_3},\;B{F_3},\;Be{F_2}$

Answer 1

To find out which molecule has zero dipole moment we need the net dipole moment of the given compounds and for that we need to check the polarity of the compounds. Generally, Polar compounds do not have zero dipole moment.

Complete answer:
In ${H_2}O$ molecule, the oxygen atom being much more electronegative than the hydrogen atom will cause the delocalisation of electrons towards it. However, the presence of a lone pair on an oxygen atom causes the ${H_2}O$ molecule to be in a bent shape according to VSEPR theory. Therefore, the individual bond dipole moments do not cancel out each other and there is a net dipole moment.

In $C{O_2}$ molecules, the geometry is linear. At the center, there is a carbon atom with oxygen on both sides. As oxygen is more electronegative than carbon, the electron cloud is shifted towards oxygen and both oxygens pull the electron cloud from each side with the same force canceling out each other. So the net dipole moment will be zero.

In $CC{l_4}$ molecule, each carbon atom is bonded with $4$ chlorine atoms from the corner of the tetrahedral geometry. As Chlorine is more electronegative than carbon, the electron cloud is shifted towards Chlorine and each Chlorine atom will pull the electron cloud from each side with the same force, so the net dipole moment will be zero just like $C{O_2}$.

In $CHC{l_3}$ molecule, each carbon atom is bonded with $3$ chlorine atoms and $1$ Hydrogen atom in a tetrahedral geometry. Electronegativity of Carbon and hydrogen is more or less equal so there will be no dipole in the $C - H$ bond while Chlorine is more electronegative than carbon so the electron cloud will be shifted towards Chlorine and each Chlorine atom will pull the cloud from each side with the same force resulting in some dipole moment. So, the net dipole moment will not be zero.

In $B{F_3}$ molecule, the structure is a trigonal planar in which the ${\text{3 }}B - F$ bonds are placed at $120^\circ$ angle to each other. As Fluorine is more electronegative than boron and the three bonds lie in one plane so the dipole moments of these bonds cancel each other giving a net dipole moment equal to zero.

In the $N{H_3}$ molecule, the structure is tetrahedral so nitrogen being more electronegative than hydrogen will pull the electron cloud towards itself. Hence, the net dipole moment is not zero.
In $Be{F_2}$ molecules, the geometry is linear. At the center, there is a carbon atom with fluorine on both sides. As fluorine is more electronegative than carbon the electron cloud is shifted towards fluorine and both fluorine atoms pull the electron cloud from each side with the same force canceling the dipole formed. So the net dipole moment will be zero.

Hence, out of the given compounds, $Be{F_2}$,; $B{F_3}$,; $C{O_2}$ and $CC{l_4}$ will have zero dipole moment.

Note:
Dipole moment is the measure of the polarity of a bond between two atoms in a molecule. It arises when there is a difference in electronegativity between the two atoms. The dipole moment of water is $1.84\,D$, the dipole moment of $CHC{l_3}$ is $1.01\,D$