Problem 17 - Chemistry The Science in Context

Question

Arrange the following molecular geometries in order of increasing bond angle: (a) trigonal planar; (b) octahedral; (c) tetrahedral.

Step-by-Step Solution

Answer

Question: Arrange the given molecular geometries in order of increasing bond angle: (a) Trigonal planar, (b) Octahedral, (c) Tetrahedral. Answer: Octahedral, Tetrahedral, Trigonal Planar

Step 1: Understanding molecular geometries

First, let's briefly review the geometries of the given molecular shapes: (a) Trigonal planar: This shape has three atoms surrounding a central atom, with all four atoms lying in the same plane. The bond angle for a trigonal planar shape is 120°. (b) Octahedral: This shape has six atoms surrounding a central atom, forming a regular octahedron. The bond angle for an octahedral shape is 90°. (c) Tetrahedral: This shape has four atoms surrounding a central atom, forming a regular tetrahedron. The bond angle for a tetrahedral shape is 109.5°.

Step 2: Comparing bond angles

Now that we have the bond angles for each of the three given molecular shapes, we can compare them and arrange them in order of increasing bond angle: 90° (octahedral) < 109.5° (tetrahedral) < 120° (trigonal planar)

Step 3: Writing the final answer

Having compared the bond angles, we can now state the arrangement of the given molecular geometries in order of increasing bond angle: Octahedral, Tetrahedral, Trigonal Planar

Key Concepts

Trigonal Planar Geometry

In chemistry, trigonal planar geometry is a model that comes into play when a molecule consists of three atoms bonded to a central atom, all residing in the same plane. Picture a flat triangular base, where the central atom sits at the center, and the three surrounding atoms are at the vertices of an equilateral triangle.

When it comes to bond angles, trigonal planar molecules boast an angle of 120° between each pair of bonds. This angle is a result of the atoms spreading out as far as they can from each other to minimize repulsion between the electrons involved in the bonding. Due to this arrangement, molecules with trigonal planar geometry are typically associated with symmetry and uniformity, leading to certain advantageous properties, such as how they interact with light and other molecules.

Substances like boron trifluoride (BF₃) are classic examples of this geometry in action and help demonstrate why understanding molecular shapes is not just academic – it has real-world implications in material science and the synthesis of chemicals.

Octahedral Geometry

Octahedral geometry occurs when six atoms or groups of atoms are symmetrically arranged around a central atom, forming what can be visualized as two pyramids base-to-base. Each corner of the two opposing triangular bases points to a vertex where one of the six atoms resides.

The term 'octahedral' comes from the eight faces of the shape formed by this arrangement, though there are six atoms involved in regards to geometry. Now, for bond angles: in an ideal octahedral molecule, each bond angle is perfectly square at 90°. This is indicative of the equidistant spacing between all the atoms surrounding the central atom, highlighting again the goal to minimize any repulsion between the electrons in the bonding regions.

Sulphur hexafluoride (SF₆) is a classic example of an octahedral molecule. These geometries are vital to understand as they are crucial in fields like coordination chemistry, where transition metals often form complex structures with octahedral shapes.

Tetrahedral Geometry

A step closer to complexity brings us to tetrahedral geometry. Picture a three-dimensional shape where a central atom is surrounded by four other atoms at the corners of a tetrahedron - a pyramid with a triangular base. This molecular shape is one of the most commonly encountered geometries in organic and inorganic chemistry.

The bond angles here are approximately 109.5°, slightly less than the trigonal planar's 120°, and this geometry allows for the spacial separation of the outer atoms, which minimizes electron repulsion. This bond angle is very specific – it's the angle at which the repulsion between the bonds is minimized to its theoretical limit for this particular arrangement.

One could consider methane (CH₄) as the poster child for tetrahedral molecules. This geometric comprehension is crucial for understanding complex chemical reactions, molecular bonding, and protein folding in biochemistry. Overlaying this geometrical insight onto various molecular domains enhances the grasp of chemical behavior and interaction.