Question

\(\mathrm{NH}_{3}\) has a net dipole moment but boron trifluoride has zero dipole moment because (1) \(\mathrm{B}\) is less electronegative than \(\mathrm{N}\) (2) \(\mathrm{F}\) is more electronegative than \(\mathrm{H}\) (3) \(\mathrm{BF}_{3}\) is pyramidal while \(\mathrm{NH}_{3}\) is planar (4) \(\mathrm{BF}_{3}\) is trigonal planar while \(\mathrm{NII}_{3}\) is pyramidal

Short answer

Option (4): \BF_{3}\) is trigonal planar while \(NH_{3}\) is pyramidal.

Step-by-step solution

Understanding Dipole Moments

A dipole moment occurs when there is a separation of charge due to the electronegativity difference between bonded atoms, resulting in a molecule having a positive end and a negative end.

Analyze the Structure of \(\bm{NH}_{3}\)

The molecule \(NH_{3}\) (ammonia) has a trigonal pyramidal shape with nitrogen at the center and three hydrogen atoms bonded to it. Nitrogen is more electronegative than hydrogen, creating a net dipole moment pointing from hydrogen to nitrogen.

Analyze the Structure of \(\bm{BF}_{3}\)

The molecule \(BF_{3}\) (boron trifluoride) has a trigonal planar shape with boron at the center and three fluorine atoms bonded to it. Fluorine is more electronegative than boron, but because of the symmetrical planar structure, the dipole moments cancel out, resulting in a net dipole moment of zero.

Comparing the Molecular Geometries

The key difference between \(NH_{3}\) and \(BF_{3}\) is their molecular geometry. \(NH_{3}\) has a trigonal pyramidal structure, which causes the dipole moments to add up. \(BF_{3}\) has a trigonal planar structure, causing the dipole moments to cancel out.

Conclusion

The explanation for the net dipole moment in \(NH_{3}\) and the zero dipole moment in \(BF_{3}\) is due to their molecular geometry. \(NH_{3}\) is pyramidal while \(BF_{3}\) is trigonal planar.

Key concepts

electronegativity

Electronegativity is the tendency of an atom to attract electrons in a chemical bond. It's a crucial concept for understanding why some molecules have dipole moments.
Atoms with higher electronegativity will attract electrons more strongly, creating a partial negative charge.
In contrast, atoms with lower electronegativity will have a partial positive charge.
For example, in the molecule NH₃ (ammonia), nitrogen is more electronegative compared to hydrogen. This causes the nitrogen to pull electron density towards itself, resulting in a dipole moment pointing from the hydrogen atoms to the nitrogen atom.
Conversely, in BF₃ (boron trifluoride), fluorine is more electronegative than boron. While this also creates individual dipoles, the symmetrical planar structure of BF₃ ensures these dipoles cancel each other out, leading to a net dipole moment of zero.

molecular geometry

Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. It's vital for predicting and explaining the dipole moments and overall behavior of molecules.
The shape of a molecule influences how individual bond dipoles interact.
For instance, NH₃ has a trigonal pyramidal geometry due to the presence of lone pairs on the nitrogen atom. This causes bond dipoles to not cancel out completely, resulting in a net dipole moment.
On the other hand, BF₃ has a trigonal planar geometry. In this arrangement, the individual bond dipoles are evenly spaced and symmetrical, leading to their cancellation and a net dipole moment of zero.

trigonal planar

Trigonal planar is a type of molecular geometry where three atoms are bonded to a central atom in a flat, triangular shape. This geometry generally arises in molecules with a central atom that has three regions of electron density and no lone pairs.
The angles between the bonds are 120 degrees, which creates a symmetrical shape.
In the case of BF₃, the boron atom is in the center with three fluorine atoms spaced evenly around it.
Because of this symmetry, the individual dipoles cancel each other out, resulting in no net dipole moment.
Therefore, understanding molecular geometry, such as trigonal planar, is critical to understanding the behavior and properties of molecules.

trigonal pyramidal

Trigonal pyramidal is another type of molecular geometry, characterized by a central atom with one lone pair and three bonded atoms.
This configuration creates a three-sided pyramid shape with the central atom at the apex.
The bond angles in a trigonal pyramidal molecule are usually around 107 degrees, slightly less than the 109.5 degrees in a perfect tetrahedral structure, due to repulsion by the lone pair.
NH₃ is an example of a molecule with trigonal pyramidal geometry. Here, the lone pair on the nitrogen atom pushes the hydrogen atoms downwards, creating a non-symmetrical shape.
Because of this geometry, the individual bond dipoles do not cancel out, resulting in a net dipole moment for the molecule.