Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following molecules: (1) \(\mathrm{XeO}_{3}\) (2) \(\mathrm{XeOF}_{4}\) (3) \(\mathrm{XeF}_{6}\) Those having same number of lone pairs on \(\mathrm{Xe}\) are (a) (2) and (3) only (b) (1) and (2) only (c) \((1),(2)\) and (3) only (d) (1) and (3) only

Step-by-Step Solution

Answer

Answer is (a) (2) and (3) only.

Step 1: Determine Valence Electrons

Xenon (Xe) belongs to Group 18 in the periodic table and has 8 valence electrons. We will consider these valence electrons to understand the number of lone pairs on each molecule.

Step 2: Analyze XeO_3

In XeO_3, xenon (Xe) forms three sigma bonds with three oxygen atoms. Since oxygen is more electronegative, it typically forms double bonds with Xe to complete its octet. Hence, XeO_3 has no lone pairs because all electrons are involved in bonding with oxygen atoms.

Step 3: Analyze XeOF_4

In XeOF_4, xenon (Xe) forms four bonds with fluorine atoms and one bond with an oxygen atom. The total of five bonds leaves one lone pair since 8 initial valence electrons minus 5 bonding electrons results in 3 electrons, which form 1.5 pairs, so rounding gives 1 lone pair due to incomplete pairs.

Step 4: Analyze XeF_6

In XeF_6, xenon (Xe) forms six bonds with fluorine atoms. Out of 8 initial valence electrons, 6 electrons are used in bonding, leaving 2 electrons or 1 lone pair.

Step 5: Compare Lone Pairs

We find that XeOF_4 and XeF_6 each have 1 lone pair on xenon, while XeO_3 has no lone pairs.

Key Concepts

Valence Electrons

Understanding valence electrons is key to predicting how atoms bond and form molecules. Valence electrons are found in the outermost shell of an atom and determine an atom's bonding behavior. Xenon ( Xe ) is in Group 18 of the periodic table, so it traditionally has 8 valence electrons. These electrons can participate in forming bonds with other atoms or can remain as lone pairs.

When analyzing a molecular structure, we account for how many of these valence electrons are used in bond formation and how many remain as lone electrons not involved in bonding. For example, xenon uses its valence electrons to form bonds with oxygen or fluorine in the compounds we are examining.

The number of valence electrons Xe starts with is a critical factor in determining both the molecular geometry and the number of lone pairs in a compound.

Lone Pairs

Lone pairs are pairs of valence electrons that are not used in chemical bonds. They influence a molecule's shape but do not directly contribute to bonding. In xenon compounds, the presence of lone pairs can add to steric strain and affect the compound's molecular geometry.

For example, in XeOF_4 and XeF_6 , xenon has one lone pair each. These lone pairs possibly influence the shape and reactivity of these compounds. Conversely, in XeO_3 , all valence electrons are used up in bonds with oxygen, leaving no lone pairs on Xe .

Identifying lone pairs involves subtracting the number of electrons involved in bonding from the total number of valence electrons. This process helps us understand how electron configurations can lead to different molecular geometries and polarities.

Xenon Compounds

Xenon compounds can defy the conventional perception of noble gases as nonreactive. Although xenon typically prefers to stay uncombined, under the right conditions, it forms stable chemical compounds with fluorine and oxygen due to high electronegativity differences.

Let's consider three important xenon compounds:

• XeO_3 forms double bonds with oxygen, utilizing all valence electrons, resulting in no lone pairs.
• XeOF_4 , distinguished by its one lone pair due to 5 sigma bonds with Xe and surrounding atoms, limits its reactivity but influences its geometry, making it a square pyramidal shape.
• XeF_6 enables the formation of six fluorine bonds, leaving behind one lone pair, which contributes to its distorted geometry.

All these unique formations are possible due to the capability of xenon atoms to expand their valence shell, accommodating more than the conventional octet rule.

Sigma Bonds

Sigma bonds are the principle type of covalent bond where two atomic orbitals overlap directly between the nuclei of two atoms. They are key to understanding how molecules like those of xenon form stable structures. In xenon compounds, the sigma bonds contribute to the molecule's overall structure and strength.

Each bond in XeO_3 , XeOF_4 , and XeF_6 is a result of the overlap of orbitals, allowing the electrons to be shared between xenon and the attached atoms.

• For XeO_3 , three sigma bonds are formed with oxygen.
• In XeOF_4 , five sigma bonds exist — four with fluorine and one with oxygen.
• Lastly, in XeF_6 , six sigma bonds with fluorine result from overlapping orbitals.

When visualizing molecular geometry, considering how sigma bonds arrange themselves is crucial. They influence the molecular shape and the angles between atoms because each sigma bond is oriented as direct and linear as possible to minimize repulsion between electron clouds.