Question

Both BF3 and NH3 have three atoms bonded to the central atom. Which of the following is the best explanation of why the geometry of these two molecules is different? A. BF3 has three bonded atoms and no lone pairs, which makes its geometry trigonal pyramidal. B. NH3 is sp 3 hybridized, while BF3 is sp 2 hybridized. C. NH3 has one lone pair. D. BF3 is nonpolar, while NH3 is polar.

Short answer

B and C explain the geometrical difference between BF3 and NH3.

Step-by-step solution

Identify the correct geometry of BF3

BF3 has three bonded atoms and no lone pairs of electrons. According to the VSEPR theory, this results in a trigonal planar geometry. Option A is incorrect because it states that the geometry of BF3 is trigonal pyramidal, which is not true.

Evaluate the hybridization of NH3 and BF3

BF3 is sp² hybridized because it has three regions of electron density around the central boron atom. NH3 is sp³ hybridized because it has four regions of electron density (three bonds and one lone pair) around the central nitrogen atom. Thus, option B is correct.

Analyze the lone pairs in NH3

NH3 has one lone pair of electrons, which influences its geometry, making it trigonal pyramidal. This makes option C also a valid explanation.

Consider the polarity of BF3 and NH3

BF3 is non-polar because it is symmetric with equal bond dipoles that cancel out. NH3 is polar due to the lone pair creating an asymmetric charge distribution. However, this does not directly explain the difference in geometry, making option D incorrect.

Conclusion

Options B and C correctly explain why the geometry of BF3 is different from NH3.

Key concepts

VSEPR theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory helps us understand the shapes of molecules. This theory is based on the principle that electron pairs around a central atom repel each other. By maximizing their distance apart, the molecule attains a specific geometry. For instance, BF3 (boron trifluoride) has three bond pairs and no lone pairs. The repulsion among these three bond pairs creates a trigonal planar shape. So, when you view BF3, you'll see that all three fluorine atoms spread out evenly around the boron, forming a flat triangle.

In contrast, NH3 (ammonia) possesses a different arrangement. It has three bond pairs and one lone pair of electrons. The lone pair is not bonded to another atom, causing greater repulsion than bonded pairs. Because of this, the molecular geometry of NH3 becomes trigonal pyramidal, a structure where the nitrogen atom is at the top with the hydrogen atoms forming a pyramid base.

hybridization

Hybridization is an important concept when learning about molecular shapes and bonding. It describes how atomic orbitals mix to form new hybrid orbitals that influence molecular geometry.

Let's break this down with BF3 and NH3 as examples. BF3 has three regions of electron density (three bonds) around boron. Boron's orbitals mix to form three sp² hybrid orbitals. This allows the molecule to form a trigonal planar shape because all three orbitals lie flat.

On the other hand, NH3 has four regions of electron density (three bonds and one lone pair). The orbitals around nitrogen hybridize to form sp³ orbitals. These sp³ hybrids push apart as much as possible, creating a trigonal pyramidal shape. This is why NH3 adopts a different geometry than BF3.

lone pairs

Lone pairs play a crucial role in determining molecular geometry. A lone pair refers to a pair of valence electrons that are not shared between atoms in a bond. In VSEPR theory, lone pairs cause more repulsion than bonded pairs because they occupy more space around the central atom.

For example, NH3 (ammonia) has one lone pair on nitrogen. This lone pair exerts a strong repulsion on the bonding pairs, altering the geometry to trigonal pyramidal from what would be a tetrahedral shape if only considering bond pairs.

BF3 (boron trifluoride), in contrast, has no lone pairs on boron. Hence, the three bond pairs remain evenly spaced out, forming a trigonal planar shape. The absence of lone pairs in BF3 removes any additional repulsion effects, leading to a planar configuration.