Question

Why can an odd-electron molecule not satisfy the octet rule?

Short answer

Odd-electron molecules can't satisfy the octet rule because there aren't enough electrons to complete eight for all atoms.

Step-by-step solution

Understanding the Octet Rule

The octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shell, resembling the electron configuration of a noble gas. This rule applies primarily to main-group elements and is central to the stability of molecules.

Definition of an Odd-Electron Molecule

An odd-electron molecule is a molecule with an odd number of electrons, meaning there is at least one electron that does not have a pair. Common examples include nitrogen monoxide (NO) and the hydroxyl radical (OH).

Applying the Octet Rule to Odd-Electron Molecules

For an odd-electron molecule, it is mathematically impossible to distribute the outer electrons such that every atom has eight electrons in its valence shell. The presence of an unpaired electron prevents the formation of complete electron pairs needed to satisfy the octet for all atoms.

Conclusion on Octet Satisfaction

Due to the presence of that single, unpaired electron, at least one atom in the molecule cannot achieve a full octet, thus violating the octet rule. This intrinsic property of odd-electron species is why they are described as free radicals and tend to be highly reactive.

Key concepts

Octet Rule

The octet rule is a fundamental concept in chemistry that governs how atoms form molecules. According to this rule, atoms tend to bond in a way that allows them to have eight electrons in their outermost shell, known as the valence shell. This configuration is similar to that of noble gases, which are considered stable and chemically inert due to their complete valence shells.

• Atoms will share, gain, or lose electrons to achieve a full octet in their valence shell.
• The octet rule applies primarily to main-group elements, which include the s-block and p-block elements on the periodic table.
• A full octet provides a stable electronic configuration, which is why molecules that satisfy this rule tend to be more stable.

However, it's important to note that exceptions to the octet rule exist, especially among elements with an odd number of electrons, as well as in molecules containing transition metals and those with expanded valence shells beyond eight electrons.

Valence Shell

The valence shell is the outermost electron shell of an atom that participates in chemical bonding. Electrons in this shell are known as valence electrons and are crucial in determining the chemical properties and reactivity of an atom.

• The number of valence electrons influences how atoms interact and bind with one another.
• For example, oxygen has six valence electrons, enabling it to form two bonds to achieve a full octet.
• Valence shells can hold a limited number of electrons, which is typically capped at eight for the main-group elements.

It's these valence electrons that determine the ability of an atom to bond and form stable molecules. In odd-electron molecules, achieving a full valence shell configuration, as dictated by the octet rule, becomes challenging due to the presence of unpaired electrons.

Free Radicals

Free radicals are atoms or molecules that contain an unpaired electron, making them highly reactive. In chemical terms, they are considered odd-electron species, with examples including nitrogen monoxide (NO) and hydroxyl radical (OH). Due to the presence of an unpaired electron, free radicals do not conform to the octet rule for all atoms.

• The unpaired electron in a free radical makes the molecule eager to participate in chemical reactions to achieve stability.
• This high reactivity of free radicals often leads them to engage in chain reactions, which are processes where the product of one reaction initiates another.
• In biological systems, free radicals can cause oxidative stress by reacting with important biomolecules like DNA, proteins, and lipids.

Understanding the behavior of free radicals helps in fields such as chemistry, biology, and medicine, where controlling free radical reactions can mitigate damage or harness their reactivity for beneficial purposes.