Question

Which of the following species is not likely to have a tetrahedral shape: (a) \(\mathrm{SiBr}_{4},(\mathrm{~b}) \mathrm{NF}_{4}^{+}\) (c) \(\mathrm{SF}_{4}\) (d) \(\mathrm{BeCl}_{4}^{2-}\) (e) \(\mathrm{BF}_{4}^{-}\) (f) \(\mathrm{AlCl}_{4}^{-} ?\)

Short answer

Option (c) \( \mathrm{SF}_{4} \) is not tetrahedral due to a lone pair causing a seesaw shape.

Step-by-step solution

Understand Tetrahedral Geometry

A molecule is considered to have a tetrahedral shape if it consists of a central atom bonded to four other atoms with bond angles close to 109.5 degrees. Typically, tetrahedral geometry results when the central atom is sp3 hybridized.

Analyze Each Option

We need to determine if each of the given species has a central atom surrounded by four atoms without lone pairs, thus having sp3 hybridization indicative of tetrahedral geometry.

Examine Each Species' Geometry

- (a) \( \mathrm{SiBr}_{4} \) has Si as a central atom with four Br atoms, no lone pairs, and sp3 hybridization. - (b) \( \mathrm{NF}_{4}^{+} \) consists of N surrounded by four F atoms, no lone pairs, and sp3 hybridization.- (c) \( \mathrm{SF}_{4} \) has S as the central atom with four F atoms but also includes a lone pair, resulting in a seesaw shape, not tetrahedral.- (d) \( \mathrm{BeCl}_{4}^{2-} \) has Be surrounded by four Cl, no lone pairs, indicating tetrahedral geometry.- (e) \( \mathrm{BF}_{4}^{-} \) has B with four F atoms, no lone pairs, and sp3 hybridization.- (f) \( \mathrm{AlCl}_{4}^{-} \) has Al surrounded by four Cl atoms, no lone pairs, and sp3 hybridization.

Identify the Non-Tetrahedral Species

Among the listed species, \( \mathrm{SF}_{4} \) is the only molecule that does not have a tetrahedral shape because the presence of a lone pair on the sulfur atom leads to a seesaw geometry instead of a tetrahedral one.

Key concepts

Tetrahedral Shape

The concept of a tetrahedral shape in molecular geometry is both fascinating and important. When we talk about a molecule having a tetrahedral shape, we're referring to a three-dimensional structure where a central atom is symmetrically bonded to four other atoms or groups. Imagine a pyramid with a triangular base, and you'll have a mental picture of a tetrahedral shape.
The bond angles in a perfectly tetrahedral molecule are about 109.5 degrees, which occurs when the central atom uses sp3 hybrid orbitals. This allows the atoms to space themselves as far apart as possible, minimizing electron repulsion.
Examples would include molecules like methane ( CH_{4} ) or ammonium ion ( NH_{4}^{+} ), where no lone pairs are present on the central atom, allowing for clean, symmetrical separation of bonded electron pairs.

Hybridization

Hybridization is a key concept in understanding molecular geometry, including the tetrahedral shape. It refers to the mixing of atomic orbitals in an atom to produce new hybrid orbitals of equivalent energy. These hybrid orbitals are used to form bonds with other atoms.
For a tetrahedral shape, the term sp3 hybridization comes into play. This involves one s orbital and three p orbitals mixing to form four equivalent sp3 hybrid orbitals. Each of these orbitals can accommodate a pair of electrons, allowing for four bonds surrounding the central atom.

• sp3 hybridization results in bond angles close to 109.5 degrees, as seen in methane.
• This is typical for central atoms with no lone pairs and four bonds, leading to a tetrahedral shape.

Understanding hybridization helps us predict and explain molecular shapes and bond characteristics.

Lone Pairs

Lone pairs are non-bonding pairs of electrons that significantly impact molecular geometry. While they do not participate in bonding, they occupy space around the central atom. This space occupation can lead to alterations in the predicted molecular shape.
In a situation where a central atom has lone pairs in addition to bonding pairs, the overall molecular shape can deviate from its standard form. The electron repulsion from lone pairs is greater than from bonding pairs, compressing bond angles and affecting the overall geometry.
For example, sulfur in SF_{4} has one lone pair. While you might expect tetrahedral geometry from the four bonding pairs, the lone pair distorts this shape into a seesaw geometry, demonstrating the influence of lone pairs.

Seesaw Geometry

Seesaw geometry is a specific molecular shape that arises when a central atom in a molecule has one lone pair and is bonded to four other atoms. This shape is an interesting deviation from a more symmetrical tetrahedral arrangement.
Seesaw geometry is named for its resemblance to the childhood teeter-totter playground equipment, with bond angles that differ from the standard 109.5 degrees seen in tetrahedral arrangements. This geometry, resulting from lone pair presence, leads to a unique arrangement where the molecule appears top-heavy or skewed.
SF_{4} is an example, where sulfur central atom is surrounded by four fluorine atoms and a lone pair, causing the seesaw structure. Understanding this helps us grasp how slight changes, like adding a lone pair, can significantly alter a molecule's geometry, emphasizing the versatility and adaptability of chemical bonding.