Oxidation State

[Celine 1F]

How do you find the oxidation state of an element?

[Julie Park 1G]

Most oxidation states of elements in molecules/compounds can be found by looking at the groups of the periodic table (PT). The oxidation number represents the # of electrons that an atom can gain, lose, or share when chemically bonding with an atom of another element.

Ex: NaCl
Na (in first group) has a +1 oxidation state (because Na will lose one electron to obtain a full valence shell)
Cl (halogen) has a -1 oxidation (because Na will gain one electron to obtain a full valence shell)
The sum of the oxidation states should always equal 0, unless the charge of the compound is indicated in the name (see next example). In addition, oxidation states of transition metals often have to be deduced based on the oxidation states of the other atoms in the molecule.

Ex: KMnO4
K (in first group) has a +1 oxidation state (because K will lose one electron to obtain a full valence shell)
O (in first group) has a -2 oxidation state (oxygen, according to the PT, will lose 2 e- to obtain a full valence shell). However, since there are x4 Oxygen, multiply -2 by 4 to get a total of -8.
You will notice that the sum of the oxidation states found so far +1 (K) + -8 (O) is -7. This means that in order for the sum to equal zero, Mn must cancel the -7 by having an oxidation state of +7.
Therefore, the oxidation states are +1(K), -2(O), and +7(Mn)

[Megan Cao 1I]

the oxidation state of a free element (H+, F2) is always zero
the oxidation number of a group 1 element is +1
the oxidation number of a group 2 element is +2
the oxidation number of a group 17 element is -1 in a binary compound
the oxidation number of a monotomic ion is equal to the charge of the ion
the oxidation of H is +1
the oxidation of O is -2 unless in a peroxide it's -1


REMEMBER: the sum of oxidation numbers in a neutral atom is 0 and the sum of oxidation numbers in a polyatomic ion is equal to the charge of the ion

[Sally Qiu 2E]

for monoatomic ions, the oxidation state is the same as the charge, so just look at the periodic table.

[Shutong Hou_1F]

Besides all the important, useful rules and hints mentioned by previous peers, when trying to decide the oxidation state of TM, we could solve the problem by knowing the charge on the molecule overall and figuring out oxidation states of all atoms other than the TM.