Which statement about oxidation number is incorrect?

None.

The correct answer is option 4. None.

Let us delve deeper into each statement about oxidation numbers to ensure complete understanding:

1. The oxidation number is the number of electrons lost (+ve) or gained (-ve) by an atom for the ionic compounds.

In ionic compounds, atoms gain or lose electrons to form ions. The oxidation number (or oxidation state) represents the effective charge of these ions:

For cations (positive ions), the oxidation number is positive and equals the number of electrons lost. For example, \(Na^+\) has an oxidation number of +1 because it has lost one electron.

For anions (negative ions), the oxidation number is negative and equals the number of electrons gained. For example, \(Cl^-\) has an oxidation number of -1 because it has gained one electron.

Conclusion: This statement is correct.

2. For covalent compounds, the oxidation number is indicated by the charge that an atom of the element would have acquired if the substance had been ionic.

In covalent compounds, electrons are shared between atoms rather than fully transferred as in ionic compounds. To assign oxidation numbers in covalent compounds:

We pretend the compound is ionic.

We assign electrons to the more electronegative atom as if it had taken the electrons completely.

For example, in water (\(H_2O\)), oxygen is more electronegative than hydrogen, so we assign all shared electrons to oxygen. As a result, oxygen gets an oxidation state of -2, and each hydrogen gets an oxidation state of +1.

Conclusion: This statement is correct.

3. Oxidation number may have integer or fractional values.

While oxidation numbers are usually integers, there are cases where they can be fractional. Fractional oxidation states typically occur in compounds with elements in mixed oxidation states or in certain polyatomic ions:

For example, in superoxides like potassium superoxide (\(KO_2\)), the oxygen molecule has an overall charge of -1. Each oxygen atom in \(O_2^-\) thus has an oxidation state of -1/2.

Another example is iron in magnetite (\(Fe_3O_4\)), where the oxidation states of iron average to fractional values, considering the mixed Fe(II) and Fe(III) states.

Conclusion: This statement is correct.

4. None.

The statement "None" implies that none of the previous statements are incorrect.

Since all previous statements (1, 2, and 3) are correct, the option "None" is also technically correct, as there are no incorrect statements provided.

Given the explanations above, it is clear that statements (1), (2), and (3) about oxidation numbers are all correct. Therefore, the statement that none of the provided statements are incorrect (option 4) is also accurate.

Hence, the answer is: (4) None.