Question

Describe the bond angles to be found in each of the following molecular structures: (a) planar trigonal, (b) tetrahedral, (c) octahedral, (d) linear.

Short answer

In each molecular structure, the bond angles are as follows: (a) Planar trigonal: all bond angles are 120°, as the atoms lie in one plane forming an equilateral triangle. (b) Tetrahedral: bond angles are approximately 109.5°, minimizing repulsion between electron pairs. (c) Octahedral: bond angles are 90° between adjacent atoms and 180° between opposite atoms. (d) Linear: the bond angle is 180°, with all atoms aligned in a straight line.

Step-by-step solution

(Planar Trigonal Bond Angles)

In a planar trigonal (or trigonal planar) molecular structure, there are three atoms bonded to a central atom, and all atoms lie in one plane. The bond angles are determined by drawing lines between the atoms and the central atom, forming an equilateral triangle. In this structure, all bond angles are equal to 120°.

(Tetrahedral Bond Angles)

In a tetrahedral molecular structure, there are four atoms bonded to a central atom. The structure is shaped like a tetrahedron, where all vertices (atoms) are equidistant from the central atom. The bond angles in this structure are approximately 109.5°, because this angle minimizes repulsion between the electron pairs around the central atom.

(Octahedral Bond Angles)

In an octahedral molecular structure, there are six atoms bonded to a central atom. The structure is shaped like an octahedron, with the central atom at its center. The bond angles in this structure are as follows: - Between any two adjacent atoms (vertices), the bond angle is 90°. - Between any two opposite atoms (vertices), the bond angle is 180°.

(Linear Bond Angles)

In a linear molecular structure, there are two atoms bonded to a central atom, and all three atoms are aligned in a straight line. In this case, the bond angle between the two atoms and the central atom is 180°.

Key concepts

Bond Angles

Bond angles are the measures of the angle formed between three atoms connected by bonds. They are crucial in determining the shape and stability of a molecule. Understanding bond angles helps in predicting molecular geometry and the behavior of the molecule in chemical reactions.

• A larger bond angle generally means less crowding among atoms and less repulsion between electron pairs.
• Knowing the bond angles allows chemists to deduce the three-dimensional arrangement of atoms in a molecule.

These angles are influenced by factors such as the size of the atoms involved, the electronic configuration, and the forces between electron pairs. The VSEPR (Valence Shell Electron Pair Repulsion) theory is a key tool used to predict bond angles and molecular geometry, based on the idea that electron pairs around a central atom tend to repel each other, arranging themselves as far apart as possible.

Planar Trigonal

The planar trigonal molecular geometry, also known as trigonal planar, is characterized by three atoms attached to a central atom, all located in the same plane. This arrangement allows the atoms to form an equilateral triangle.

• The bond angles in a planar trigonal structure are exactly 120°.
• This angle is optimal to minimize repulsion between the electron clouds of the bonds.

The planar trigonal shape often appears in molecules where the central atom has no lone pairs, such as in molecules with a central element from Group 13 of the periodic table. For example, boron in BH₃ exhibits this geometry. Understanding this configuration helps in predicting the reactivity and interactions of the molecule with other substances.

Tetrahedral Structure

The tetrahedral molecular structure features four atoms bonded to a central atom, arranged in a three-dimensional shape resembling a pyramid with a triangular base. This configuration is a result of attempting to keep the bond angles as equal as possible.

• The bond angles in a tetrahedral structure measure approximately 109.5°.
• This angle prevents electron pair repulsion among the bonds, providing stability to the molecule.

A well-known example of a tetrahedral molecule is methane (CH₄), where carbon is the central atom. The tetrahedral shape is significant in many biological molecules, including amino acids and certain enzyme structures, influencing their function and interaction.

Octahedral Structure

The octahedral molecular geometry involves six atoms symmetrically arranged around a central atom, forming an octahedron. This structure optimizes spatial arrangement to minimize electron-pair repulsion.

• Adjacent atoms have bond angles of 90° relative to each other.
• Opposite atoms, on straight axes, share bond angles of 180°.

Compounds like sulfur hexafluoride (SF₆) showcase octahedral geometry. Understanding this structure is crucial in many fields, such as chemistry and material science, because it affects the compound's physical and chemical properties. The coordination and bond strength in octahedral molecules play a role in their catalytic and electronic properties.

Linear Structure

In a linear molecular structure, atoms arrange themselves in a straight line, leading to unique spatial characteristics. Typically, two atoms bond to a central atom, which allows the molecule to have the maximum bond angle possible.

• In these structures, the bond angles are always 180°.
• The linear arrangement can be found in molecules like carbon dioxide (CO₂) and diatomic molecules such as hydrogen (H₂).

Linear geometry is often associated with molecules that have multiple bonds or a high level of molecular symmetry. This geometry affects how the molecule interacts with other molecules and can influence gases' packing in a solid state.