Problem 47 - A Complete Resource Book in Chemistry for JEE Main

Question

The shape of \(\mathrm{IF}_{7}\) molecule is (a) pentagonal bipyramidal (b) trigonal pyramidal (c) tetrahedral (d) square planar

Step-by-Step Solution

Answer

The shape of \(\mathrm{IF}_7\) is pentagonal bipyramidal.

Step 1: Determine the number of valence electrons

The iodine atom belongs to Group 17 and has 7 valence electrons. Each fluorine atom contributes 1 valence electron. Since there are 7 fluorine atoms in \(\mathrm{IF}_7\), the total count of valence electrons from fluorine is 7. Thus, the total number of valence electrons in \(\mathrm{IF}_7\) is \(7 + 7 = 14\). However, since we are only considering iodine's electrons for determining the geometry, we consider the 7 electrons iodine has.

Step 2: Apply VSEPR theory

Based on VSEPR (Valence Shell Electron Pair Repulsion) Theory, iodine's 7 valence electrons are bonded with the 7 fluorine atoms. Thus, there are 7 bond pairs and no lone pairs on the central iodine atom, which adopts a \(7\) coordinate geometry that is typical for a pentagonal bipyramidal shape.

Step 3: Identify molecular geometry

The central iodine atom makes 7 bonds and no lone pairs are present. This distribution leads to a pentagonal bipyramidal shape as it minimizes repulsion between the electron pairs.

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. By understanding the geometry of a molecule, you can predict its properties, reactivity, and polarity. The molecular geometry depends on the number of bonded atoms and lone pair electrons around the central atom.

In molecules with a single central atom, Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the molecular geometry. This theory posits that electron pairs will arrange themselves to minimize repulsion, leading to specific 3D shapes.

Different geometries include:

• Linear
• Trigonal planar
• Tetrahedral
• Trigonal bipyramidal
• Octahedral
• Pentagonal bipyramidal

By determining the molecular shape, chemists can derive important insights about how the molecule interacts with others, its phase at room temperature, and its potential role in various chemical reactions.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. These electrons are crucial because they participate in chemical bonding and reactions. For elements in the main groups, the number of valence electrons corresponds to the group number in the periodic table.

For example:

• Iodine (I) is in Group 17, so it has 7 valence electrons.
• Fluorine (F), also in Group 17, has 1 valence electron per atom.

To determine the shape of a molecule like \(IF_7\), we focus mainly on the valence electrons of the central atom, iodine in this case. In \(IF_7\), each of the 7 valence electrons forms a bond with a fluorine atom. This information helps in predicting the geometry using VSEPR theory.

Pentagonal Bipyramidal

The pentagonal bipyramidal shape is a specific molecular geometry that occurs with certain compounds where the central atom is surrounded by seven other atoms. In this structure, the atoms form a plane of five (hence 'pentagonal'), with the remaining two atoms positioned above and below this plane ('bipyramidal').

This geometry is particularly stable in molecules that have no lone pairs on the central atom, as the \(IF_7\) molecule demonstrates. Here, iodine forms bonds with seven fluorine atoms, utilizing all its valence electrons. By adopting a pentagonal bipyramidal shape, the molecule minimizes the repulsion forces between these electrons, leading to a stable structure.

Pentagonal bipyramidal is significantly different from other common geometries because it represents a higher coordination number, which is the number of other atoms directly bonded to the central atom. Understanding this geometry is vital for chemists when predicting molecular behavior and reactivity.