Problem 19 - Chemistry The Central Science In Si Units

Question

In which of these molecules or ions does the presence of nonbonding electron pairs produce an effect on molecular shape? (a) \(\mathrm{CO}_{2},(\mathbf{b}) \mathrm{CH}_{2} \mathrm{Br}_{2},(\mathbf{c}) \mathrm{OF}_{2},\) (d) \(\mathrm{BCl}_{3}\), (e) \(\mathrm{SF}_{6}\).

Step-by-Step Solution

Answer

Nonbonding electron pairs affect the shape of OF2.

Step 1: Understanding Nonbonding Electron Pairs

Nonbonding electron pairs, also known as lone pairs, can influence the molecular geometry by occupying space and repelling bonded atoms. This repulsion often affects the overall shape of the molecule.

Step 2: Analyzing Molecule CO2

The central atom in CO2 is carbon, which forms double bonds with two oxygen atoms. There are no lone pairs on the carbon atom, so the shape of CO2 remains linear without distortion due to nonbonding pairs.

Step 3: Analyzing Molecule CH2Br2

In CH2Br2, the central atom is carbon, which forms single bonds with two hydrogen atoms and two bromine atoms. Carbon has no lone pairs, meaning the shape (tetrahedral) is not affected by nonbonding pairs at the central atom.

Step 4: Analyzing Molecule OF2

Oxygen in OF2 acts as the central atom bonded to two fluorine atoms. Additionally, oxygen has two nonbonding electron pairs. These lone pairs cause a bent shape due to their repulsion, which distorts the molecular geometry from linear.

Step 5: Analyzing Molecule BCl3

Boron in BCl3 is surrounded by three chlorine atoms with no lone pairs on boron. Thus, the molecular shape is a planar trigonal, and no effect is observed from nonbonding pairs as boron doesn't possess any.

Step 6: Analyzing Molecule SF6

Sulfur is the central atom in SF6 and forms six equivalent bonds with fluorine atoms. Since no lone pairs are present on sulfur, the geometry is octahedral, and there is no distortion due to nonbonding electron pairs.

Key Concepts

Nonbonding Electron Pairs

Nonbonding electron pairs, also known as lone pairs, are valence electrons that are not involved in bonding with other atoms. These electron pairs can strongly influence the shape of a molecule. Understanding the role of nonbonding electron pairs is crucial because they exert repulsive forces on the other bonded atoms in a molecule. This repulsion:

• Changes the angles between the bonds.
• Creates unique molecular shapes.

For example, in the molecule OF2, oxygen has two lone pairs that repel the bonded fluorine atoms, creating a bent shape. These lone pairs take up more space around the central atom compared to bonded pairs because they are only bounded by one nucleus instead of two, intensifying their repulsive effect.

Molecular Shape

The molecular shape or geometry of a molecule is defined by the positions of the nuclei of the atoms in space. The arrangement is determined by the number of bonds and lone pairs on the central atom. Key shapes to remember include:

• Linear
• Tetrahedral
• Trigonal planar
• Bent
• Octahedral

Molecules such as CO2 and BCl3 have symmetrical shapes like linear and trigonal planar, respectively, because they lack lone pairs on the central atoms. However, in the presence of lone pairs, like in OF2, the shape changes to accommodate the added repulsion, resulting in a bent molecular shape. The shape adopted is the one that minimizes the repulsion between electron pairs.

Lone Pairs Effect

Lone pairs are unshared electrons that are crucial in defining the corners of a molecule's geometry, due to their ability to repel bonded electron pairs. This effect changes the perceived shape and angular displacement of bonds. Lone pairs possess greater repulsive power in comparison to bonding electron pairs. This increase in repulsion

• reduces the bond angle between bonded pairs,
• pushes bonded atoms closer together,

modifying the expected geometric shapes. As a result, molecules like OF2, diverge from expected linear shapes to a bent shape because the lone pairs on the oxygen push the fluorine atoms closer together by occupying more space around the oxygen atom. This effect is absent in molecules such as CO2 and SF6 where nonbonding electron pairs are not present on the central atom, thus preserving the predicted geometries.