Problem 206 - A Complete Resource Book in Chemistry for JEE Main

Question

The molecular shapes of \(\mathrm{SF}_{4}, \mathrm{CF}_{4}\), and \(\mathrm{XeF}_{4}\) are (a) different with 1,0 and 2 lone pairs of electrons on the central atoms respectively (b) different with 0,1 and 2 lone pairs of electrons on the central atoms respectively (c) the same with 1,1 and 1 lone pair of electrons on the central atoms respectively (d) the same with 2,0 and 1 lone pair of electrons on the central atoms respectively

Step-by-Step Solution

Answer

(a) Each molecule has a different shape with 1, 0, and 2 lone pairs.

Step 1: Determine Shape and Lone Pairs for SF4

Identify the molecular geometry and the number of lone pairs on the central atom for \( \text{SF}_4 \). Using VSEPR theory, \( \text{SF}_4 \) has a total of 10 electrons around the sulfur (4 are shared with fluorine and 2 remain as a lone pair). This molecule adopts a seesaw shape with 1 lone pair on the sulfur atom.

Step 2: Determine Shape and Lone Pairs for CF4

Identify the molecular geometry and the number of lone pairs on the central atom for \( \text{CF}_4 \). Carbon shares all four of its valence electrons with the four fluorine atoms, leaving no lone pairs. This results in a tetrahedral shape with 0 lone pairs on the central carbon atom.

Step 3: Determine Shape and Lone Pairs for XeF4

Identify the molecular geometry and the number of lone pairs on the central atom for \( \text{XeF}_4 \). Xenon shares 4 of its 8 valence electrons with the fluorine atoms, leaving 2 lone pairs. This results in a square planar shape with 2 lone pairs on the xenon atom.

Step 4: Analyze Answer Choices

Based on the molecular shapes and lone pairs identified: \( \text{SF}_4 \) has 1 lone pair, \( \text{CF}_4 \) has 0 lone pairs, and \( \text{XeF}_4 \) has 2 lone pairs. Therefore, the description that matches these findings is (a) different with 1,0 and 2 lone pairs of electrons on the central atoms respectively.

Key Concepts

VSEPR theory

VSEPR theory, pronounced as "vesper," stands for Valence Shell Electron Pair Repulsion theory. It's a model used by chemists to predict the shape of individual molecules. The concept is simple yet powerful: electron pairs around a central atom arrange themselves to minimize repulsion. This means they will spread out as much as possible, to maintain the lowest energy—and most stable—configuration.
This arrangement helps us understand molecular geometry, explaining why certain shapes form based on the number of bonds and lone pairs of electrons. Whether we're dealing with linear, trigonal planar, or more complex shapes, VSEPR theory provides a clear framework for understanding molecular shapes.

lone pairs

Lone pairs are pairs of valence electrons not involved in chemical bonding. In molecular geometry, they are vital to consider because they occupy space and influence the shape of a molecule.

• For example, in a molecule like SF4, the lone pair on the sulfur atom pushes the bonds slightly, creating a seesaw shape.
• In XeF4, the two lone pairs on xenon result in a square planar shape, as their presence alters the ideal geometry.

These non-bonding electrons play a crucial role in determining the geometry, as they exert repulsion just like bonded pairs, but often more strongly.

molecular shapes

Molecular shapes can be quite diverse, and they depend heavily on the number of bonded pairs and lone pairs surrounding the central atom. The shape directly affects the molecule's properties and behavior.

• Linear, trigonal planar, tetrahedral, and octahedral are some of the basic arrangements.
• Complex shapes arise due to lone pairs, such as the seesaw shape in SF4 or the square planar shape in XeF4.

Understanding these shapes allows chemists to predict physical and chemical properties of substances, such as polarity and reactivity.

central atom

The central atom is typically found at the center of a molecule, bonded to surrounding atoms, and is pivotal in determining the molecule's shape. Its valence electrons participate in bonding with other atoms' electrons.
It's the atom around which the geometry of the molecule is determined. For example, in CF4, the carbon atom is central and surrounded symmetrically by four fluorine atoms, forming a perfect tetrahedral shape. Meanwhile, in SF4 and XeF4, sulfur and xenon are the central atoms, with lone pairs affecting their final geometry.

SF4

The molecule sulfur tetrafluoride (\(SF_4\)) is an intriguing example of molecular geometry affected by lone pairs. It consists of a sulfur atom at its center, bonded to four fluorine atoms. However, it has one lone pair on sulfur.

• This lone pair causes the shape to deviate from a perfect tetrahedral to a seesaw configuration.
• Such shapes occur as lone pairs occupy more space than bonding pairs, pushing the bonded atoms into less regular formations.

Understanding SF4’s geometry helps illustrate how lone pairs influence molecular shapes.

CF4

Carbon tetrafluoride (\(CF_4\)) has a more straightforward shape due to the lack of lone pairs on the central atom. Here, carbon is bonded to four fluorine atoms, effectively utilizing all its valence electrons for chemical bonds.

• This molecule forms a tetrahedral shape, with bond angles of 109.5 degrees.
• The absence of lone pairs allows the bonds to arrange themselves evenly around the carbon atom, creating a balanced, symmetrical shape.

CF4 serves as an excellent example of how the absence of lone pairs leads to classical geometric shapes.

XeF4

Xenon tetrafluoride (\(XeF_4\)) is a fascinating molecule, particularly because it involves a noble gas as the central atom. Xenon, typically unreactive, can form bonds under certain conditions, as seen in XeF4 where it is surrounded by four fluorine atoms.

• With two lone pairs also present on xenon, the molecule adopts a square planar shape.
• The lone pairs sit opposite each other, minimizing repulsion while allowing the bonded fluorines to align in a flat, square-like configuration.

Studying XeF4 helps underscore how even elements known for stability can form complex molecular geometries under specific conditions.