Question

The molecules \(\mathrm{NH}_{3}\) and \(\mathrm{BF}_{3}\) have the same general formula \(\left(\mathrm{AB}_{3}\right)\) but different shapes. a. Find the shape of each of the above molecules. b. Provide more examples of real molecules that have the same general formulas but different shapes.

Short answer

a. The shape of \(\mathrm{NH}_{3}\) molecule is trigonal pyramidal and the shape of \(\mathrm{BF}_{3}\) molecule is trigonal planar. b. Examples of molecules with the same general formulas but different shapes include SO3 and SF4 (both AB3) with trigonal planar and see-saw shapes, and H2O and BeH2 (both AB2) with bent and linear shapes, respectively.

Step-by-step solution

Apply VSEPR Theory on \(\mathrm{NH}_{3}\)

Determine the number of lone pairs and bonding pairs of electrons around the central atom (Nitrogen). Nitrogen has 5 valence electrons, and it forms bonds with 3 hydrogen atoms which contribute 1 electron each. That means there are 3 bonding pairs and 2 non-bonding electrons (1 lone pair). According to VSEPR theory, the shape of the molecule will be trigonal pyramidal.

Apply VSEPR Theory on \(\mathrm{BF}_{3}\)

Determine the number of lone pairs and bonding pairs of electrons around the central atom (Boron). Boron has 3 valence electrons, and it forms bonds with 3 fluorine atoms which contribute 1 electron each. That means there are 3 bonding pairs and 0 non-bonding electrons (0 lone pairs). According to VSEPR theory, the shape of the molecule will be trigonal planar.

List the shapes of the given molecules

Based on our calculations using the VSEPR theory: - The shape of \(\mathrm{NH}_{3}\) molecule is trigonal pyramidal - The shape of \(\mathrm{BF}_{3}\) molecule is trigonal planar

Provide examples of molecules with the same general formulas but different shapes

Here are some examples of molecules that have the same general formulas but different shapes: 1. SO3 and SF4: Both have a general formula AB3. SO3 is a trigonal planar molecule (with 0 lone pairs on the central sulfur atom), while SF4 is a see-saw shaped molecule (with 1 lone pair on the central sulfur atom). 2. H2O and BeH2: Both have a general formula AB2. H2O is a bent-shaped molecule (with 2 lone pairs on the central oxygen atom), while BeH2 is a linear molecule (with 0 lone pairs on the central beryllium atom).

Key concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It's influenced by both the bonding pairs of electrons (which form bonds between atoms) and the lone pairs of electrons (which are not shared between atoms). VSEPR (Valence Shell Electron Pair Repulsion) Theory helps us predict the molecular geometry by assuming that electron pairs will arrange themselves to minimize the repulsion between them.

Understanding the shape of a molecule is crucial because it determines many of the molecule’s properties, such as reactivity, polarity, phase of matter, color, magnetism, and biological activity. For example, though both water (H₂O) and carbon dioxide (CO₂) contain the same number of atoms, their different molecular geometries lead to dissimilar properties, such as water being a polar liquid at room temperature and CO₂ being a nonpolar gas.

Trigonal Pyramidal

A trigonal pyramidal shape is one where a central atom is connected to three surrounding atoms at the corners of a pyramid with a triangular base. The central atom typically has one lone pair, which pushes the bonding atoms slightly closer together, creating a pyramid shape rather than a flat plane.

The nitrogen atom in ammonia (NH₃) is an example of a trigonal pyramidal molecule. It has three bonding pairs forming a pyramid and one lone pair at the apex, which distorts the geometry from a perfect tetrahedral shape. The presence of the lone pair gives the molecule a slight polarity, making NH₃ a polar molecule with distinct physical and chemical properties.

Trigonal Planar

In trigonal planar molecular geometry, a central atom is surrounded by three other atoms, all lying in a single plane. This geometric arrangement occurs when there are three bonding pairs of electrons and no lone pairs on the central atom.

The boron atom in boron trifluoride (BF₃) is an example of a trigonal planar molecule. All three bonding pairs of electrons spread out as far as possible to minimize repulsion, resulting in bond angles of 120 degrees and a flat, planar shape. This symmetric arrangement makes BF₃ a nonpolar molecule, which profoundly affects its behavior and interactions with other molecules.

Lone Pairs and Bonding Pairs

Lone pairs and bonding pairs are the two types of valence electron pairs found in molecules, and they play a vital role in determining molecular geometry. Bonding pairs are shared between two atoms, forming a bond, while lone pairs are unshared electrons that are found only on one atom.

The VSEPR Theory suggests that both lone pairs and bonding pairs repel each other and that the molecule’s shape adjusts to minimize this repulsion. Lone pairs particularly have a stronger repulsive force than bonding pairs, often resulting in bond angle reductions from the ideal geometry. For instance, in water (H₂O), the two lone pairs on the oxygen atom force the hydrogen atoms closer together, resulting in a bent shape rather than linear.