Question

Which of the following is an electron deficient molecule? (a) \(\mathrm{C}_{2} \mathrm{H}_{6}\) (b) \(\mathrm{NH}_{3}\) (c) \(\mathrm{BeCl}_{2}\) (d) \(\mathrm{SiCl}_{4}\)

Short answer

The electron-deficient molecule is \\(\mathrm{BeCl}_{2}\\).

Step-by-step solution

Understanding Electron Deficiency

An electron-deficient molecule is one that does not have a complete octet. Typically, this involves molecules where the central atom does not have enough electrons to complete its valence shell with bonds alone.

Analyzing \\(\mathrm{C}_{2}\mathrm{H}_{6}\\)

Ethane, \(\mathrm{C}_{2}\mathrm{H}_{6}\), consists of two carbon atoms. Each carbon atom forms four single bonds (two with hydrogen and one with the other carbon), satisfying the octet rule for both carbon atoms. Thus, \(\mathrm{C}_{2}\mathrm{H}_{6}\) is not electron deficient.

Analyzing \\(\mathrm{NH}_{3}\\)

Ammonia, \(\mathrm{NH}_{3}\), consists of a nitrogen atom bonded to three hydrogen atoms. Nitrogen has five valence electrons and can form three single bonds with hydrogen to achieve a full octet, making \(\mathrm{NH}_{3}\) non-electron-deficient.

Analyzing \\(\mathrm{BeCl}_{2}\\)

Beryllium chloride, \(\mathrm{BeCl}_{2}\), involves a central beryllium atom which only has two valence electrons and forms two bonds with chlorine atoms, resulting in only four electrons around the beryllium. This does not complete the octet, making \(\mathrm{BeCl}_{2}\) an electron-deficient molecule.

Analyzing \\(\mathrm{SiCl}_{4}\\)

Silicon tetrachloride, \(\mathrm{SiCl}_{4}\), involves a central silicon atom bonded to four chlorine atoms. Silicon has four valence electrons and forms four single bonds, achieving a full octet with the shared electrons from chlorine. Hence, \(\mathrm{SiCl}_{4}\) is not electron deficient.

Conclusion

Among the given options, \(\mathrm{BeCl}_{2}\) is the electron-deficient molecule as it does not have a complete octet around the central beryllium atom.

Key concepts

Octet Rule

In chemistry, the octet rule is a principle that atoms tend to form compounds in ways that give them eight valence electrons, equating to the electron configuration of a noble gas. This rule guides the prediction of molecular bonding patterns. Atoms attempt to reach an octet in order to achieve maximum stability, similar to noble gases.
For many elements, especially non-metals, achieving an octet can occur by sharing electrons through chemical bonds. This often results in molecules like water (\(\mathrm{H}_2\mathrm{O}\)) and methane (\(\mathrm{CH}_4\)). However, there are exceptions. Some elements, such as hydrogen, lithium, and beryllium, do not follow this rule strictly due to their inability to accommodate six or more additional electrons.
Electron-deficient molecules, such as \(\mathrm{BeCl}_2\), are unable to fulfill this rule due to their limited number of valence electrons. Despite having fewer electrons than the octet rule prescribes, these molecules can still be stable through other mechanisms, such as forming bonds with lone pair electrons from other atoms.

Chemical Bonding

Chemical bonding refers to the forces holding atoms together in molecules. Understanding these bonds is essential to comprehend molecular structures. The three primary types of chemical bonds are ionic, covalent, and metallic bonds.

• Ionic bonds occur when electrons are transferred from one atom to another, resulting in a positive ion and a negative ion that attract each other.
• Covalent bonds involve the sharing of electron pairs between atoms. This type of bond is often seen in non-metal elements and is the framework for many organic compounds. Molecules like ammonia (\(\mathrm{NH}_3\)) and water are prime examples.
• Metallic bonds are formed by the attraction between free-floating valence electrons and the positively charged metal ions. This type gives metals their characteristic properties such as conductivity and malleability.

Electron-deficient molecules, such as \(\mathrm{BeCl}_2\), often don't have enough electrons to form full covalent or ionic bonds. This can lead to bonds that are weaker or involve sharing additional electrons in atypical ways.

Molecular Structure

The structure of a molecule significantly influences its properties and reactivity. Molecular structure involves the spatial arrangement of atoms and the bonds between them, providing insights into the molecule's characteristics.
Beryllium chloride (\(\mathrm{BeCl}_2\)) provides a relevant example, as it is linear with beryllium in the center flanked by two chlorine atoms. Here, the lack of a complete octet for beryllium alters its reactive nature and influences its interactions with other molecules.

• The geometry of a molecule is determined by the number and arrangement of its atoms and bonds.
• Coordination numbers indicate how many atoms or ions immediately surround a central atom in a complex or structure, influencing the molecule's stability and reactivity.
• Molecular structures also determine how a molecule interacts with electric fields and how it absorbs and emits light.

By analyzing the molecular structure, chemists can predict physical properties and potential reactivity, aiding in fields such as pharmacology and materials science. Understanding these elements is key to harnessing the potential of complex molecules, even those that seem deficient by octet standards.