Question

Which of the following is not a rule for calculating oxidation number?

A. For ions, oxidation number is equal to the charge on the ions
B. The oxidation number of oxygen is -2 in all compounds
C. The oxidation number of fluorine is -1 in all compounds
D. Oxidation number of hydrogen is +1 except in binary hydrides of alkali metals and alkaline earth metals where it is -1

Answer 1

Hint: We know that, oxidation state for an atom is the number of electrons that an atom loses or gains to result in a chemical bond. The value of the oxidation number is zero, positive or negative. The highest oxidation state is achieved by platinum, that is, 10.

Complete step by step answer:
Let’s discuss the given options in detail.
Option A says the oxidation number of ions is equal to its charge. This statement is correct. For example, oxidation state of bromine is -1, -1 is the charge of bromide ion $\left( {{\rm{B}}{{\rm{r}}^ - }} \right)$. So, A is a rule to calculate oxidation number.
Option B says the oxidation number of oxygen is -2 in all compounds. Oxygen is the element of atomic number 8. Oxygen possesses three oxidation states. In peroxide, oxygen atom exists in -1 oxidation state, that is, \[{{\rm{O}}_{\rm{2}}}^ - \], in \[\]${{\rm{F}}_{\rm{2}}}{\rm{O}}$, the oxidation state of oxygen is +2 and in all other compounds, the oxidation state of oxygen is -2.

Option C says oxidation number of fluorine is -1 in all compounds. This statement is correct. Fluorine is the only halogen which possesses only one oxidation state that is, -1. So, option C is a rule to calculate oxidation number.

Option D says the oxidation state of hydrogen is +1 except in case of binary hydrides of alkali metals and alkaline earth metals where it’s oxidation state is -1.
Therefore, option C is not a rule to calculate oxidation number. Hence, the correct answer is option C.


Note:
Some other rules to be remembered while calculating oxidation state are,

1) If a compound exists in elemental form (only one type of atoms present), the oxidation number of the element is always zero. For example, in ${{\rm{H}}_{\rm{2}}}$ oxidation state of H is 0.

2) For a neutral compound, summation of oxidation states of all atoms in the compound is equal to zero.