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The rules for determining oxidation states

The oxidation state of an atom that is part of a molecule or has a certain chemical form, such as an ion in an aqueous solution, is the electrical charge it would have if its bonds with other elements were ionic . In electrochemical reactions, electron transfer occurs; mass and charge are conserved, but it is necessary to know which atoms are oxidized (lose electrons when combining) and which atoms are reduced (gain electrons) during the reaction. Oxidation states tell us how many electrons each atom loses or gains in an electrochemical reaction. Let us now see the rules that allow us to determine the oxidation states.

Determination of oxidation states: rules

  • Regarding the nomenclature of compounds , the cation is written first in the chemical formula, followed by the anion. In the chemical formula for table salt NaCl (sodium chloride) sodium is an Na + cation and chlorine is a Cl – anion .
  • The oxidation state of a free element is always 0 . So, for example, the oxidation state of oxygen atoms in air (O2 ) and metallic mercury (Hg) is 0.
  • The sum of the oxidation states of all the atoms in a neutral compound is 0 , and the sum of the oxidation states in a polyatomic ion is equal to the ion’s charge ; for example, the sum of the oxidation states of SO 4 2-  is -2.
  • The oxidation state of a monatomic ion is equal to the charge on the ion . In the table salt example, the oxidation state of Na +  is +1.
  • In the case of hydrogen, in most cases its oxidation state is +1 , except when it is part of metal hydrides such as CaH 2 , in which its oxidation state is -1.
  • In the case of oxygen, in most cases its oxidation state is -2 , but there are exceptions such as oxygen difluoride (OF 2 ) in which the oxidation state of oxygen is +2, and peroxides, such as barium peroxide (BaO 2 ), in which the oxidation state of oxygen is -1.
  • A general rule in the combination of elements is called the octet rule.. Because the most stable electronic configuration of an atom is that of the noble gases, when atoms combine they tend to have eight electrons in their outer shell. Therefore, the oxidation state of an element is related to its location on the periodic table of elements. If it belongs to group IA, its oxidation state will be +1 and if it belongs to group IIA, its oxidation state will be +2, since these will be the electrons that it will tend to give up when combined so that its external electronic layer resembles the noble gas more close on the table. At the other end of the table, the oxidation state of a group VIIA element is -1, except when that element is combined with one that has a higher electronegativity.

Font

Petrucci, RH, Herring, FG Pardo, C., Iza Cabo, N.  General chemistry  (1st edition in Spanish) . Prentice Hall. 2003.