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We are to find why ${\mathrm{CCl}}_4$is nonpolar whereas ${\mathrm{PCl}}_3$ is polar.

Each molecule's bonding orbitals are arranged in a tetrahedron; the only difference is that ${\mathrm{CCl}}_4$has enough atoms linked to the carbon to make the molecule completely tetrahedral.
Each chlorine in ${\mathrm{CCl}}_4$ is slightly negative, yet the molecule's symmetry means that the molecule is non-polar.
The steric number of phosphorus in ${\mathrm{PCl}}_3$is $4$much as carbon in ${\mathrm{CCl}}_4$However, four atoms encircle carbon.
The phosphorus in ${\mathrm{PCl}}_3$has a lone pair and three data-custom-editor="chemistry" $\mathrm{Cl}$ surrounding it.The tetrahedron cannot be completed because there are insufficient bound atoms.
The phosphorus atoms are at the top of the ${\mathrm{PCl}}_3$ molecule, while the chlorine atoms are at the bottom.