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Question Video: Determining the Molecule That Contains Polar Bonds but Is Not a Polar Molecule Chemistry

Which of the molecules Br₂, HF, CHCl₃, CCl₄, and NH₃ contains polar bonds but is not a polar molecule?

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Video Transcript

Which of the molecules Br2, HF, CHCl3, CCl4, and NH3 contains polar bonds but is not a polar molecule?

Polar bonds exist between some atoms that have different electronegativity values. The electron density is high around the more electronegative atom and low around the other bonded atom.

Chemists usually calculate the absolute difference of electronegativity values to determine if a bond will be polar or not. 𝐸 one and 𝐸 two are the electronegativity values of the atoms engaging in a bond. Bonds are polar when their atoms have a difference of electronegativity values higher than 0.4 and less than approximately 1.7 or 1.8.

The bromine molecule does not have any polar bonds. Because it has atoms with the same electronegativity value, its electron density is shared equally about the two covalently bonded atoms. Bromine cannot be the correct answer to this question because it does not have any polar bonds.

Hydrogen fluoride has a polar bond because it has atoms of different electronegativity values. The bond has an absolute electronegativity difference of 1.78, which falls approximately between the range for polar bonds. Because this molecule contains only one bond and that bond is polar, the molecule is polar overall and is not the answer to this question.

We can calculate that CHCl3 contains three polar bonds. NH3, or ammonia, has three polar bonds as well. We can use dipole arrows to show the polarity of the bonds. These molecules are polar because the dipole moment of one polar bond does not cancel out the dipole moment of other polar bonds. CHCl3 and NH3 are polar molecules.

CCl4, or carbon tetrachloride, contains only polar bonds. The molecule, however, is nonpolar because it has symmetrical geometry. These four equivalent carbon–chlorine bonds generate four dipole moments. These dipole moments cancel each other, and the molecule has no overall polarity. Carbon tetrachloride is the only listed molecule that has polar bonds but is nonpolar. Therefore, the correct answer is CCl4.

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