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240 CHEMISTRY in a covalent bond belongs entirely to more bonding state of oxygen but this number electronegative element. would now be a positive figure only. It is not always possible to remember or 4. The oxidation number of hydrogen is +1, except when it is bonded to metals in binary make out easily in a compound/ion, which element is more electronegative than the compounds (that is compounds containing other. Therefore, a set of rules has been two elements). For example, in LiH, NaH, and CaH , its oxidation n er is –1 formulated to determine the oxidation 2 number of an element in a compound/ion. 5. In all its compounds, fluorine has an If two or more than two atoms of an element oxidation n er o –1 Ot er alogens (Cl, are present in the molecule/ion such as Br, and I) also have an oxidation number 2– o –1, en t e occ r as alide ions in Na 2S2O3/Cr 2 7 the oxidation number of the atom of that element will then be the average their compounds. Chlorine, bromine and of the oxidation number of all the atoms of iodine when combined with oxygen, for example in oxoacids and oxoanions, have that element. We may at this stage, state the rules for the calculation of oxidation number. positive oxidation numbers. These rules are: 6. The algebraic sum of the oxidation number of all the atoms in a compound must be 1. In elements, in the free or the uncombined state, each atom bears an oxidation zero. In polyatomic ion, the algebraic sum of all the oxidation numbers of atoms of number of zero. Evidently each atom in the ion must equal the charge on the ion. H2, O2, C2 , 3 , 4 ,8S , Na, Mg, Al has the Thus, the sum of oxidation number of three oxidation number zero. oxygen atoms and one carbon atom in the 2. For ions composed of only one atom, the carbonate ion, (CO 32– st eq al –2 oxidation number is equal to the charge By the application of above rules, we can on the ion. Thus Na ion has an oxidation 2+ 3+ find out the oxidation number of the desired nu–ber of +1,2–g ion, +2, Fe ion, +3, element in a molecule or in an ion. It is clear Cl ion, –1, O ion, –2 and so on n t eir that the metallic elements have positive compounds all alkali metals have oxidation number of +1, and all alkaline earth metals oxidation number and nonmetallic elements have positive or negative oxidation number. haveanoxidationnumberof+2.Aluminium The atoms of transition elements usually is regarded to have an oxidation number of display several positive oxidation states. The +3 in all its compounds. highest oxidation number of a representative 3. The oxidation number of oxygen in most element is the group number for the first co o nds is –2 Ho ever, e co e two groups and the group number minus 10 across two kinds of exceptions here. (following the long form of periodic table) for One arises in the case of peroxides and the other groups. Thus, it implies that the superoxides, the compounds of oxygen in highest value of oxidation number exhibited which oxygen atoms are directly linked to by an atom of an element generally increases each other. While in peroxides (e.g., 2 2 across the period in the periodic table. In the Na2O 2, each oxygen atom is assigned an third period, the highest value of oxidation oxidation n er o –1, in s eroxides number changes from 1 to 7 as indicated (e.g., KO2, RbO 2 each oxygen atom is below in the compounds of the elements. assigned an oxidation n er o –( ) A term that is often used interchangeably The second exception appears rarely, i.e. with the oxidation number is the oxidation when oxygen is bonded to fluorine. In state. Thus in CO 2, the oxidation state of such compounds e.g., oxygen difluoride carbon is +4, that is also its oxidation number (OF ) and dioxygen difluoride (O F ), the and similarly the oxidation state as well 2 2 2 oxygen is assigned an oxidation number as oxidation n er o ox gen is – 2 is of +2 and +1, respectively. The number implies that the oxidation number denotes the assigned to oxygen will depend upon the oxidation state of an element in a compound. Reprint 2026-27