Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Among \(\mathrm{BF}_{3}, \mathrm{NF}_{3}, \mathrm{PH}_{3}, \mathrm{IF}_{3}, \mathrm{IF}_{5}\) and \(\mathrm{SF}_{4}\), the number of species having the same number of lone pair of electrons on the central atom is

Step-by-Step Solution

Answer

Three species have the same number of lone pairs on the central atom.

Step 1: Identifying the Central Atom and Valence Electrons

First, determine the central atom for each molecule and calculate the total number of valence electrons for these atoms. For example, in \(\mathrm{BF}_3\), the central atom is boron, which has 3 valence electrons. Similarly, identify central atoms and their valence electrons for \(\mathrm{NF}_3\), \(\mathrm{PH}_3\), \(\mathrm{IF}_3\), \(\mathrm{IF}_5\), and \(\mathrm{SF}_4\).

Step 2: Drawing the Lewis Structures

Draw the Lewis structure for each molecule to distribute electrons into bonds. Make sure to follow the octet rule when possible. For \(\mathrm{BF}_3\), boron forms three bonds with fluorine atoms, using all its 3 valence electrons and leaving no lone pairs. Follow a similar process for other molecules.

Step 3: Calculating Lone Pairs on Central Atom

For each molecule, calculate the number of lone pairs on the central atom by considering the remaining valence electrons after bond formation. For \(\mathrm{NF}_3\), nitrogen forms 3 bonds with fluorine, with one pair of lone electrons left, resulting in one lone pair. Repeat this process for each molecule.

Step 4: Comparing the Number of Lone Pairs

Compare the number of lone pairs across all the molecules. Group molecules based on having the same number of lone electron pairs on the central atom.

Step 5: Counting the Number of Species with Matching Lone Pairs

Identify how many different groups you have, each with a unique number of lone pairs. Count how many molecules share the same number of lone pairs on their central atom.

Key Concepts

Lone Pair Electrons

Lone pair electrons refer to the electrons present on an atom that are not involved in chemical bonding. These electrons are important because they can affect the shape and polarity of a molecule. When determining the number of lone pair electrons, you must first consider the total number of valence electrons that the atom possesses. After forming bonds, any remaining electrons are classified as lone pairs.

For example, in the molecule ** NF₃**:

• Nitrogen is the central atom with five valence electrons.
• It forms three bonds with three fluorine atoms, using six electrons (one from each participating atom).
• Counting nitrogen’s valence electrons, two electrons are left unbonded, forming one lone pair.

It is crucial to identify lone pairs because they can occupy space and influence the geometry predicted by **VSEPR theory** (Valence Shell Electron Pair Repulsion Theory). This interaction helps predict the 3D shape of molecules, providing insights into their physical and chemical properties.

Lewis Structures

Lewis structures are a simple way to represent molecules in which the valence electrons are depicted as dots around an atom. They help in visualizing the bonding in a molecule and determining how the electrons are distributed among the atoms. To create an accurate Lewis structure:

• Identify the total number of valence electrons in the molecule.
• Select a central atom, usually the one that is least electronegative, and distribute the electrons around it.
• Arrange the remaining atoms around the central atom, forming bonds by sharing electrons.

In BF₃, boron is the central atom with three valence electrons. It forms three single bonds with three fluorine atoms. Each fluorine atom contributes one electron, leading to a total of six bonding electrons forming three B-F single bonds, with no lone electrons remaining on the boron atom.

Deducing the correct Lewis structure ensures every atom follows the **octet rule** (where applicable) and helps identify any lone pairs present, aiding in predicting molecular geometry and reactivity.

Central Atom Valence Electrons

Understanding central atom valence electrons is essential in predicting bonding and structure of molecules. Valence electrons are those present in an atom’s outermost shell and are crucial because they participate in chemical bonding. For each molecule mentioned, identifying the central atom and counting its valence electrons is the first step. Here’s how you might approach it:

• ** BF₃**: Boron (central atom) has 3 valence electrons.
• ** NF₃**: Nitrogen (central atom) has 5 valence electrons.
• ** PH₃**: Phosphorus (central atom) has 5 valence electrons.
• ** IF₃**: Iodine (central atom) has 7 valence electrons.
• ** IF₅**: Iodine (central atom) has 7 valence electrons.
• ** SF₄**: Sulfur (central atom) has 6 valence electrons.

After counting these electrons, you distribute them in a way that satisfies the octet rule, drawing the Lewis structure accordingly. Properly identifying and using central atom valence electrons facilitates the prediction of potential lone pairs and helps us understand the resultant molecular structure and properties.