Problem 200 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is an electron deficient molecule? \(\quad\) [2005] (a) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{PH}_{3}\) (c) \(\mathrm{B}_{2} \mathrm{H}_{6}\) (d) \(\mathrm{SiH}_{4}\)

Step-by-Step Solution

Answer

The electron deficient molecule is \\mathrm{B}_2\\mathrm{H}_6 (Diborane).

Step 1: Understanding Electron Deficiency

Electron deficiency occurs in compounds with incomplete octet structures, which typically happen in molecules with elements that require additional electrons to complete their valence shell.

Step 2: Analyzing Each Option

We examine each molecule: (a) C_2H_6 (Ethane) - All carbon and hydrogen atoms in ethane have complete electron configurations with no deficiency. (b) PH_3 - Phosphorus in PH_3 has a complete octet. (c) B_2H_6 (Diborane) - Boron in B_2H_6 is electron deficient because each boron atom forms bonds with only 3 valence electrons available, not enough to reach a stable octet. (d) SiH_4 - Silicon in SiH_4 has a complete octet.

Step 3: Final Selection of Electron Deficient Molecule

Since B_2H_6 (Diborane) is the compound where each boron atom cannot complete its octet, B_2H_6 is determined to be electron deficient.

Key Concepts

Incomplete Octet

In chemistry, the term "incomplete octet" refers to the concept where certain elements cannot have the eight electrons typically required to achieve a stable electronic configuration. Most atoms strive to have eight electrons in their valence shell, known as the "octet rule." However, some elements, like boron and aluminum, often form compounds where they possess fewer than eight valence electrons, leading to the "incomplete octet" status. This scenario is characteristic of electron-deficient molecules.

• Boron and aluminum are common examples of elements that show incomplete octet behavior.
• Molecules with incomplete octets are often trying to share or acquire additional electrons through bonding with other atoms.

Understanding incomplete octets is essential for explaining the structures of certain molecules, especially those in group 13 of the periodic table, like boron.

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. These are the electrons involved in forming bonds between atoms. When atoms bond, they either share, gain, or lose valence electrons to achieve a more stable electron arrangement typically conforming to the octet rule.

Knowing the number of valence electrons helps predict how an atom will bond. For example:

• Carbon has four valence electrons, which allows it to form four covalent bonds.
• Boron has three valence electrons, which it uses to form bonds but leaves it electron deficient.

Valence electrons are central to understanding how atoms interact and form stable chemical compounds.

Chemical Bonding

Chemical bonding involves the interaction of atoms to achieve stability, often requiring atoms to complete their valence shells. There are several types of chemical bonds, with covalent and ionic being the most common.

• Covalent Bond: Atoms share pairs of valence electrons, common in organic molecules.
• Ionic Bond: Atoms transfer electrons to achieve full valence shells, resulting in positively and negatively charged ions.

Diborane ( B_2H_6 ) is a fascinating molecule with a unique bonding scenario where electron sharing doesn't fulfill typical notions of valence needs. In diborane, boron's bonding involves creating 3-center-2-electron bonds, which are less common and contribute to its electron-deficient classification.

Diborane

Diborane ( B_2H_6 ) is a notable electron-deficient molecule featuring boron and hydrogen atoms. In its structure, two boron atoms form bonds that involve only three valence electrons per boron atom, which is inadequate for a complete octet. As a result, diborane exhibits a unique structure that includes uncommon "banana bonds," formally known as "3-center-2-electron bonds."

This peculiar structure occurs because:

• Two hydrogen atoms form bridges between the boron atoms.
• These bridge bonds share electrons among three atoms rather than two, which is different from typical covalent bonds.

Understanding diborane is critical for grasping electron deficiency and unconventional bonding patterns in chemistry.

Molecular Structure

The molecular structure of a compound helps us understand the spatial arrangement of its atoms and the type of chemical bonds. It provides invaluable insight into the properties and behavior of the molecule. For electron-deficient molecules like diborane, structure elucidation is essential to comprehend its bonding nature.

Diborane has a distinct molecular structure featuring:

• Boron atoms connected by bridging hydrogen atoms, forming a unique bonding arrangement.
• The presence of symmetric "banana bonds" that create a stable, although electron-deficient, configuration.

Examining its structure helps chemists deduce not just stability, but also reactivity and other physical properties.