Question

How many of the following molecules possess dipole moments: $B{H_3},C{H_4},PC{l_5},{H_2}O,HF,{H_2}?$

Answer 1

The bond dipole moment is the moments but property of the chemical bond that is used to measure the polarity of the bond within a molecule. The occurrence of bond dipole is when there is a separation of positive and negative charges. It is given by the formula $\mu = \delta d$ and depends upon the charges on the atoms that form the bonds and the distance between them.
The dipole moment of a molecule is also based on the shape of the molecule. Symmetrical shapes with equal charges usually cancel each other dipole moments thus given the molecule a net dipole moment of zero.

Complete step by step solution:
The separation of charges in a system leads to a dipole moment. Not only ionic compounds, covalently bonded compounds can also develop a dipole moment because a dipole moment is a property based on the relative electronegativity of the bonded atoms.
The molecular dipole moment is the vector sum of the individual dipole moments of each bond, thus let us compare the bonding structures of the given molecules.
We know that $B{H_3}$ forms a trigonal planar structure and hydrogen is more electronegative than boron, thus the individual dipole moments will move away from the central atom and cancels out each other. Thus making $B{H_3}$ a non-polar molecule owing to its symmetric structure.
Similarly, $C{H_4}$ forms a regular tetrahedral structure and has very small electronegativity difference between the atoms that cancels out each other due to their directional vectors. Thus $C{H_4}$ is non polar.
The $PC{l_5}$ is made up of polar $P - Cl$ bonds that form a trigonal bipyramidal molecular which is arranged symmetrically thus the $PC{l_5}$ molecule is nonpolar.
${H_2}O$ is a polar compound with a bent shape, which is the reason for the polarity of the compound.
$HF$ is a polar molecule owing to higher electronegativity difference and no other bonds to cancel the polarity thus formed.
${H_2}$ is a diatomic molecule and hence it is non-polar due to charges being equally distributed among the atoms?
Thus, the atoms that possess dipole moments are ${H_2}O,HF$ .

Note:
We know that every molecule in the above list has individual bond dipole moments but the vector sum cancels out each other due to the symmetric shape or may be because the bonds are in the same plane.