Rules for Assigning Oxidation Number

Introduction

The total number of electrons that an atom gains or loses to establish a chemical connection with another atom is known as oxidation number, also known as oxidation state. Oxidation-reduction reactions have oxidation numbers that indicate each atom’s ability to accept, donate, or share electrons. 

For example, the ion of iron Fe3+ has an oxidation number of +3 because it can take on three electrons to form a chemical bond. The sum of the oxidation numbers in an electrically neutral substance is zero; For example, in hematite (Fe2O3), the oxidation number of the two iron atoms (+6 in total) balances the oxidation number of the three oxygen atoms (6) in an electronically neutral substance.

In some compounds, certain elements have the same oxidation number; Fluorine, for example, has the oxidation number 1 in all of its compounds. Others, particularly non-metals and transition elements, can have a wide range of oxidation numbers; For example, nitrogen can have any oxidation number between -3 (as in ammonia, NH3) and +5(as in nitric acid, HNO3). 

Typically, inorganic chemistry nomenclature represents elements in more than one oxidation state by placing a roman numeral in parentheses after the element name. For example, iron (II) chloride (FeCl2) and iron (III) chloride (FeCl3) are represented by roman numerals.

Rules for Assigning an Oxidation Number

The transfer of electrons is involved in electrochemical reactions. Mass and charge are conserved when balancing these reactions, but it is important to know which atoms are oxidised and which atoms are reduced. Each atom’s oxidation number is used to track how many electrons it loses or gains. The following rules are used while calculating the oxidation number.

• First, it is mandatory to write the cation in a formula, followed by the anion. The H in NaH is H-, whereas the H in HCl is H+
• The oxidation number of a free element is always zero
• For example, The oxidation numbers of the atoms in He and N2 are 0
• For instance, Mono-atomic ions have the same charge as their oxidation number
• The oxidation number of Na+, for example, is +1, whereas that for N3- is -3
• The oxidation number of hydrogen is usually +1
• Compounds with elements that are less electronegative than hydrogen, like, in the case of CaH2, the oxidation number of hydrogen is -1
• Compounds, where oxygen has an oxidation number of -2, are the most common compounds
• OF2 is an exception because fluorine is more electronegative than oxygen, and BaO2 is also an exception because the peroxide ion’s structure is [O-O]2-
•  The compound with an element of group IA has an oxidation number of +1
• Whenever a compound contains an element from Group IIA, its oxidation number is 2
• The oxidation number of a Group VIIA element in a compound is -1 except when it is paired with a higher electronegativity element
• For example, In the case of HCl, Cl has an oxidation number of -1, whereas, in HOCl, it shows an oxidation number of +1
• A neutral substance does not have an atom having oxidation numbers greater than zero
• In the case of a polyatomic ion, its charge is equal to the sum of its oxidation numbers
• For example, the oxidation number for SO42-, is -2

These are the principles that provide an alternative method to define oxidation and reduction in terms of oxidation numbers. For instance, a substance is oxidised when its oxidation number increases—vice-versa in the case of reduction.

Oxidation Number Calculation of an Atom

The amount of electrons an atom/ion has gained or lost compared to the neutral atom is known as its oxidation number or state. Group 1, 2, and 3 electropositive metal atoms lose a particular amount of electrons and have a constant positive oxidation number. More electronegative atoms in molecules acquire electrons from a less electronegative atom, resulting in negative oxidation states. 

The number of electrons lost or gained determines the oxidation state’s numerical value. The oxidation number or oxidation state of an atom or ion is determined by: (i) Adding the constant oxidation states of other atoms/molecules/ions linked to it, and ii) Equating the overall oxidation state of a molecule or ion to its total charge.

Oxidant

An oxidising agent (often referred to as an oxidiser or an oxidant) is a chemical species that tends to oxidise other substances.

Conclusion

Depending on how much oxidation or reduction an atom has undergone, its oxidation number will be positive or negative. The transfer of electrons between chemical entities is the driving force for chemical change in oxidation-reduction processes. There are ten rules for determining oxidation numbers that are listed above.