Question

Explain the meaning of the following terms. (a) expanded octet (b) resonance (c) unshared electron pair (d) odd-electron species

Short answer

Question: Provide brief explanations for the following terms related to molecular structure and chemical bonding: (a) expanded octet, (b) resonance, (c) unshared electron pair, and (d) odd-electron species. Answer: (a) An expanded octet is a molecule in which an atom has more than eight electrons in its valence shell, typically seen in period 3 or higher elements. (b) Resonance refers to a molecule that cannot be represented by a single Lewis structure but is best described by a hybrid of two or more contributing structures. (c) An unshared electron pair, or lone pair, is a pair of valence electrons not shared between atoms in a molecule and plays a significant role in molecular shape and reactivity. (d) Odd-electron species are molecules or ions with an odd number of electrons, resulting in an unpaired electron, making them highly reactive and short-lived.

Step-by-step solution

(a) Expanded octet

An expanded octet is a term used to describe a molecule in which an atom (usually from period 3 or higher) has more than eight electrons in its valence shell. This situation occurs because these atoms have access to empty d-orbitals, which can accommodate additional electron pairs. Examples of molecules with expanded octets include phosphorus pentafluoride (PF5) and sulfur hexafluoride (SF6).

(b) Resonance

Resonance is a phenomenon in which a molecule cannot be adequately represented by a single Lewis structure. Instead, it is best described by a mixture of two or more contributing structures, known as resonance structures or resonance forms. These resonance forms usually differ in the arrangement of electrons, while the positions of atoms remain the same. The actual molecule is considered to be a hybrid of these resonance forms, with its overall structure and properties being a weighted average of all possible structures. An example of a molecule that exhibits resonance is benzene (C6H6), which has two resonance forms consisting of alternating single and double carbon-carbon bonds.

(c) Unshared electron pair

An unshared electron pair, also known as a lone pair or nonbonding electron pair, is a pair of valence electrons that are not shared between atoms in a molecule. These electrons belong exclusively to one atom and do not participate in chemical bonding. Unshared electron pairs play a significant role in determining the shape of molecules and the reactivity of certain chemical species. For example, in water (H2O), each oxygen atom has two unshared electron pairs, which contribute to its bent molecular geometry and its ability to act as a both a Bronsted-Lowry acid and base.

(d) Odd-electron species

An odd-electron species is a molecule or ion that contains an odd number of electrons, leading to the presence of an unpaired electron. These species are often highly reactive and short-lived due to the presence of the unpaired electron, which makes them more likely to participate in chemical reactions to achieve a more stable configuration. Examples of odd-electron species include free radicals, such as the hydroxyl radical (OH•) and nitrogen monoxide (NO•), and certain ions like nitric oxide (NO+).