Question

Which statement about oxidation number is incorrect? (1) The oxidation number is the number of electrons lost (+ve) or gained \((-\mathrm{vc})\) by an atom for thc ionic compounds. (2) For covalent compounds the oxidation number is indicated by the change that an atom of clcment would have acquired if the substance would have been ionic. (3) Oxidation number may have integer or fractional values. (4) Nonc.

Short answer

Statement 3 is incorrect.

Step-by-step solution

Understanding Oxidation Numbers

Oxidation numbers represent the number of electrons an atom gains, loses, or shares when it forms compounds.

Analyzing Statement 1

Statement 1 correctly states that the oxidation number is the number of electrons lost (positive) or gained (negative) by an atom in ionic compounds.

Analyzing Statement 2

Statement 2 correctly applies to covalent compounds, where oxidation numbers are assigned as if the compounds were ionic.

Analyzing Statement 3

Statement 3 is incorrect. Oxidation numbers are almost always integers. Fractional oxidation numbers are rare exceptions.

Final Analysis

Since statement 3 is incorrect and all other statements are correct, statement 3 is the answer to the question.

Key concepts

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons from one to another. This transfer creates ions: positively charged ions (cations) and negatively charged ions (anions). Typically, metals lose electrons to become cations, and non-metals gain electrons to become anions. These oppositely charged ions attract each other and form a compound.
For example:

• Sodium chloride (NaCl) forms when sodium (Na) loses one electron and becomes Na⁺, and chlorine (Cl) gains one electron to become Cl^-.

In ionic compounds, oxidation numbers indicate how many electrons are lost or gained by each atom. It helps in predicting the formula and properties of the compound. Understanding ionic compounds is crucial to grasping redox reactions and various chemistry principles.

Covalent Compounds

Covalent compounds form when atoms share electrons rather than transferring them. These shared electrons allow each atom to achieve a stable electronic configuration or 'octet'.
In a covalent compound:

• Atoms share electrons equally (nonpolar covalent bonds) or unequally (polar covalent bonds).
• An example is water (H₂O), where oxygen shares electrons with hydrogen atoms.

Although actual sharing occurs, oxidation numbers are assigned as if the compound were ionic. This hypothetical scenario makes it easier to understand the splitting of electrons:

• Consider the hypothetical charges an atom would have in an ionic state to determine the oxidation number.

By mastering covalent bonding, students can further explore molecular geometry, polarity, and reactivity.

Fractional Oxidation Numbers

Oxidation numbers are almost always integers, representing whole amounts of electron transfer. However, in some rare cases, fractional oxidation numbers arise. This scenario typically occurs in compounds featuring electrons shared among multiple atoms.
Example case:

• Consider iron in magnetite (Fe₃O₄). Here, iron displays an average oxidation state of +8/3, reflecting a mix of Fe²⁺ and Fe³⁺.

Important aspects to note about fractional oxidation numbers:

• They are exceptions and not the rule.
• Usually signify a resonance structure or mixed valency state.

Understanding fractional oxidation numbers helps students deal with complex inorganic chemistry problems and gives insight into advanced topics like electronic configurations and solid-state chemistry.