Question

Predict the electron pair geometry, the molecular shape, and the bond angle for a carbon tetrabromide molecule, \(\mathbf{C B r}_{4}\), using VSEPR theory.

Short answer

CBr₄ has a tetrahedral shape with bond angles of 109.5°.

Step-by-step solution

Identify Valence Electrons

Carbon (C) has 4 valence electrons, and each bromine (Br) contributes 1 valence electron to form bonds. In total, carbon tetrabromide has the electron configuration for making 4 single bonds.

Determine Steric Number

The steric number is determined by the number of atoms bonded to the central atom (C) and the number of lone pairs on the central atom. In CBr₄, there are 4 atoms bonded to C and 0 lone pairs, so the steric number is 4.

Predict Electron Pair Geometry

With a steric number of 4 and no lone pairs, the electron pair geometry around carbon is tetrahedral as per VSEPR theory.

Determine Molecular Shape

For a compound with a tetrahedral electron pair geometry and all positions occupied by bonded atoms, the molecular shape is also tetrahedral.

Estimate Bond Angle

In a tetrahedral geometry, the bond angles between atoms are approximately 109.5°.

Key concepts

Carbon Tetrabromide

Carbon tetrabromide, often represented by the chemical formula \( \text{CBr}_4 \), is a compound where a central carbon atom is bonded to four bromine atoms. Each bromine atom forms a single bond with the carbon through sharing a pair of electrons. The carbon atom contributes four valence electrons, with each bromine atom contributing one, leading to the formation of four covalent bonds. This complete sharing results in a molecule that has no free electron pairs on the carbon, which is crucial for understanding its geometric configuration.

Carbon tetrabromide is part of the larger family of halomethanes, which are derivatives of methane. In these compounds, hydrogen atoms are replaced by halogen atoms, in this case, bromine. This substitution results in differences in properties such as solubility, reactivity, and boiling points.

Electron Pair Geometry

The electron pair geometry of a molecule provides a glimpse into the arrangement of electron clouds around the central atom. In carbon tetrabromide, this geometry is crucial for predicting the shape of the molecule as a whole. As per the Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs will arrange themselves as far apart as possible to minimize repulsion.

In CBr₄, the steric number is 4, calculated from the total number of bonded atoms and lone pairs on the carbon atom. Given that there are no lone pairs, the electron pair geometry is tetrahedral. This means that the four pairs of bonding electrons are positioned at the vertices of an imaginary tetrahedron centered on the carbon atom.

• This geometric arrangement leads to compound stability, as the electron clouds are optimally spaced.
• Such space distribution affects the molecular shape, which in turn influences physical properties and reactivity.

Molecular Shape

The molecular shape of a compound is a three-dimensional representation of how atoms are spatially arranged within the molecule. For carbon tetrabromide, the molecular shape mirrors its electron pair geometry. Because all four valence electrons in carbon form bonds with bromine atoms, there are no lone pairs to alter the arrangement of atoms.

According to VSEPR theory, this results in a symmetrical tetrahedral shape. This symmetry is pivotal as it influences how the molecule interacts with other compounds and its behavior in various chemical environments.

• The tetrahedral shape ensures that the bromine atoms are spaced as far apart as possible, reducing repulsive forces.
• This configuration impacts the molecule’s polarity, making CBr₄ nonpolar overall despite the polar character of the C-Br bonds.

Bond Angle

Bond angles in a molecule are determined by the geometric arrangement of bonds around a central atom. For carbon tetrabromide, these angles are a direct outcome of its tetrahedral electron pair geometry. The optimal bond angle in a perfect tetrahedral shape, like that of CBr₄, is approximately 109.5 degrees.

This bond angle is a result of the position of the four bromine atoms, which are evenly spread around the central carbon atom. By maintaining these angles, the molecule achieves an ideal distribution of electron clouds, minimizing repulsion and leading to stability.

• Any deviation from these angles can result in increased electron pair repulsion, destabilizing the molecule.
• This precise angle contributes to the molecule’s overall shape, influencing physical attributes like melting and boiling points.

Understanding bond angles is crucial as they explain much about the physical and chemical behavior of a molecule.