Question

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

Short answer

(a) \(\mathrm{B}_{2} \mathrm{H}_{6}\) is electron deficient.

Step-by-step solution

Understand Electron Deficiency

An electron-deficient molecule is a compound that does not have enough electrons to be able to form the typical electron pair bonds between atoms. These molecules often have an incomplete octet.

Analyze Each Option

Examine each option to identify if it has an incomplete electron configuration.- Option (a) \(\mathrm{B}_{2} \mathrm{H}_{6}\): Boron has only 3 valence electrons and can only form 3 covalent bonds. In \(\mathrm{B}_{2} \mathrm{H}_{6}\), or diborane, each boron forms multi-center bonds, not typical electron pair bonds.- Option (b) \(\mathrm{SiH}_{4}\): Silicon forms 4 bonds with hydrogen, completing its octet.- Option (c) \(\mathrm{PH}_{3}\): Phosphorus forms 3 single bonds with hydrogen, having a complete octet with a lone pair remaining.- Option (d) \(\mathrm{C}_{2} \mathrm{H}_{6}\): Carbon forms a full octet with four bonds around each atom.

Identify Electron Deficient Molecule

Since \(\mathrm{B}_{2} \mathrm{H}_{6}\) forms multi-center bonds because each boron atom does not have a full octet, it is electron deficient. The other molecules have complete octets for each central atom.

Key concepts

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom. These electrons play a crucial role in chemical bonding, as they are the electrons involved in forming bonds between atoms. The number of valence electrons an atom has can determine how it interacts with other atoms. For example:

• Boron has three valence electrons, which limits it to forming three single covalent bonds naturally.
• Silicon, with four valence electrons, typically forms four bonds to achieve stability.

The valence electrons help dictate how atoms bond and what kind of structures they form. Understanding the number of valence electrons can help predict how a molecule will behave chemically. By analyzing the valence electrons in a molecule such as \(\mathrm{B}_{2} \mathrm{H}_{6}\), it's evident why boron ends up being electron-deficient.

Chemical Bonding

Chemical bonding refers to the interaction between atoms that allows them to form molecules. There are several types of bonds, but covalent bonds are common in molecules like \(\mathrm{B}_{2} \mathrm{H}_{6}\).

• Covalent bonds occur when atoms share pairs of valence electrons to achieve a more stable electron configuration.
• Some molecules, like diborane (\(\mathrm{B}_{2} \mathrm{H}_{6}\)), form unique bonds such as multi-center bonds. This means that the bonding involves more than two atoms sharing electrons, commonly occurring in electron-deficient molecules.

The chemical bonding in diborane is quite unusual. Each boron atom doesn't follow the typical covalent bonding arrangements due to its electron deficiency. Instead, it resorts to forming multi-center bonds, which are necessary to compensate for the lack of valence electrons available to each boron atom.

Octet Rule

The octet rule is a fundamental principle in chemistry that suggests atoms tend to form molecules or compounds in which they possess eight electrons in their outer shell. This arrangement is associated with maximum stability, mimicking the electron configuration of noble gases. The rule guides our understanding of why certain molecules like \(\mathrm{SiH}_{4}\) are stable:

• Silicon can stabilize itself by forming four covalent bonds, fulfilling the octet rule.
• For electron-deficient molecules such as \(\mathrm{B}_{2} \mathrm{H}_{6}\), the atoms don't achieve a complete octet, leading to unexpected bonding patterns.

When atoms can't satisfy the octet rule through normal bonding, they find alternative means, such as forming multi-center bonds as seen in electron-deficient molecules. This requirement for stability is a core reason for the diversity of chemical compounds and their properties.