Problem 20 - Chemistry: The Central Science

Question

In which of the following molecules can you confidently predict the bond angles about the central atom, and for which would you be a bit uncertain? Explain in each case. (a) \(\mathrm{H}_{2} \mathrm{~S}_{\text {, }}\) (b) \(\mathrm{BCl}_{3}\), (c) \(\mathrm{CH}_{3} \mathrm{I}_{,}\)(d) \(\mathrm{CBr}_{4}\), (e) \(\mathrm{TeBr}_{4}\).

Step-by-Step Solution

Answer

The bond angles in BCl3, CH3I, and CBr4 can be confidently predicted, as they have trigonal-planar and tetrahedral molecular geometries with bond angles of 120 degrees and 109.5 degrees, respectively. However, there is uncertainty in predicting bond angles for H2S and TeBr4 due to the presence of lone pairs which causes deviation from the ideal angles in their molecular geometries.

Step 1: Determine the steric number and molecular geometry for each molecule

(a) H2S: Steric number: 2 (bonded atoms) + 2 (lone pairs) = 4 Molecular geometry: Tetrahedral (2 bonding pairs, 2 lone pairs). (b) BCl3: Steric number: 3 (bonded atoms) + 0 (lone pairs) = 3 Molecular geometry: Trigonal-planar (3 bonding pairs, 0 lone pairs). (c) CH3I: Steric number: 4 (bonded atoms) + 0 (lone pairs) = 4 Molecular geometry: Tetrahedral (4 bonding pairs, 0 lone pairs). (d) CBr4: Steric number: 4 (bonded atoms) + 0 (lone pairs) = 4 Molecular geometry: Tetrahedral (4 bonding pairs, 0 lone pairs). (e) TeBr4: Steric number: 4 (bonded atoms) + 1 (lone pair) = 5 Molecular geometry: Trigonal-bipyramidal (4 bonding pairs, 1 lone pair).

Step 2: Determine bond type for each molecule

(a) H2S: Polar-covalent bond (b) BCl3: Polar-covalent bond (due to electronegativity difference) (c) CH3I: Polar-covalent bond (d) CBr4: Polar-covalent bond (due to electronegativity difference) (e) TeBr4: Polar-covalent bond

Step 3: Certainty of bond angles prediction

(a) H2S: Uncertain, as the bond angle may deviate due to the presence of lone pairs causing repulsion. (b) BCl3: Confident, the bond angle in a trigonal-planar molecule is 120 degrees. (c) CH3I: Confident, the bond angle in a tetrahedral molecule is 109.5 degrees. (d) CBr4: Confident, the bond angle in a tetrahedral molecule is 109.5 degrees. (e) TeBr4: Uncertain, as the bond angles may deviate from ideal angles due to the presence of a lone pair in the trigonal-bipyramidal arrangement. In summary, the bond angles in BCl3, CH3I, and CBr4 can be confidently predicted, while there is uncertainty in predicting bond angles for H2S and TeBr4 due to the presence of lone pairs.

Key Concepts

Steric Number

The steric number is a key concept in understanding the molecular geometry of a molecule. It refers to the total number of bonded atoms plus the number of lone pairs around a central atom. To find the steric number:

• Add the number of atoms directly bonded to the central atom.
• Add the number of lone pairs on the central atom.

For example, in H₂S, the steric number is 4 (2 hydrogen atoms and 2 lone pairs). This calculation helps you predict the molecule's shape. It's crucial for determining the spatial arrangement of atoms, which impacts bond angles and molecule stability.

Bond Angles

Bond angles are the angles formed between three atoms across at least two bonds. They play a significant role in the shape and properties of a molecule. Bond angles are affected by:

• Lone pair repulsions, which can decrease bond angles.
• The molecular geometry defined by the steric number.

For example, in a tetrahedral geometry like CH₃I, bond angles are typically around 109.5 degrees. However, lone pairs, as seen in H₂S, can cause deviations from these ideal angles. Knowing the bond angles helps predict how a molecule interacts with other molecules and its reactivity.

Lone Pairs

Lone pairs are pairs of valence electrons that are not shared with other atoms in a molecule. These electron pairs are confined to the atom and cause repulsion against bonding pairs. This repulsion leads to an adjustment of bond angles and the overall geometry. Some effects of lone pairs include:

• Reducing bond angles due to their higher repulsion compared to bonded pairs.
• Altering the shape of the molecule, making it asymmetrical.

An illustrative case is TeBr₄, where the presence of a lone pair yields a trigonal-bipyramidal shape with modifications, causing uncertainties in bond angle predictions.

Trigonal Bipyramidal

Trigonal bipyramidal is one of the molecular geometries categorized under the VSEPR theory. In this shape:

• There are five positions around the central atom.
• Five atoms or groups distribute into two types of positions: axial and equatorial.
• The steric number is 5.

TeBr₄ is an example with four bonded atoms and one lone pair, fitting this geometry. The lone pair usually occupies an equatorial position due to lower repulsion, but causes variations in bond angles, making them hard to predict accurately.

Polar Covalent Bond

A polar covalent bond occurs when two atoms with different electronegativities share electrons unequally. This results in a dipole moment due to the partial positive and negative charges created. Polar covalent bonds are common in:

• Molecules with significant electronegativity differences between bonded atoms.
• Intermolecular interactions affecting boiling and melting points.

Taking BCl₃ as an example, the difference in electronegativity between boron and chlorine creates polar covalent bonds. Though the molecule is often considered nonpolar due to its symmetric trigonal-planar shape, understanding the nature of these bonds helps explain the molecule's physical and chemical properties.