The oxidation state, also known as oxidation number of an atom is the hypothetical formal charge said to be acquired by that atom while making bonds with other atoms.
As per the latest definition in the IUPAC Gold Book, the oxidation state of an atom denotes its charge after an ionic approximation of its heteronuclear bonds, and it is nearly interchangeable with the term 'oxidation number'.
The oxidation state of an atom is a fundamental concept in chemistry, reflecting the distribution of electron density over the atoms in ionic as well as covalent compounds.
The oxidation state can take on positive, negative, or zero values. It may be an integral value or a fractional number (especially the average oxidation state).
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Determining an atom's oxidation number involves scrutinizing the distribution of electrons within a compound, typically guided by rules governing electron sharing or transfer in bonds. Considerations include factors such as the electronegativities of bonded atoms and the electron count involved in bond formation.
In ionic compounds, the process is straightforward: the oxidation numbers are determined by the number of electrons transferred. Atoms gaining electrons acquire negative oxidation numbers, whereas those losing them acquire positive oxidation numbers.
Within covalent bonds, electrons are assigned to the more electronegative atom, while the less electronegative atom is assigned positive oxidation states corresponding to the apparent loss of electrons.
Rule 1: Free Elements: In their elemental form, atoms have an oxidation number of zero.
For example, oxygen (O2) and chlorine (Cl2) molecules have an oxidation number of zero each.
Rule 2: Fluorine: Fluorine always has an oxidation number of -1 in compounds.
Rule 3: Group 1 & 2 Metals: In compounds, Group 1 metals (such as sodium, potassium) always have an oxidation number of +1, while Group 2 metals (such as magnesium, calcium) always have an oxidation number of +2.
Rule 4: Oxygen: The oxidation state of oxygen is -2 in most of its compounds, such as H2O, SO2, P2O3, CO2, H2SO4, etc.
Exceptions include peroxides (O22-) where oxygen has an oxidation number of -1.
Rule 5: Hydrogen: The oxidation state of hydrogen is +1 in most of its compounds, like H2O, CH4, NH3, H2S, PH3, HCl, etc.
However, it's -1 when combined with less electronegative atoms, e.g., LiH, CsH, CaH2, etc.
Rule 6: Halogens: Halogens other than fluorine show -1 oxidation state in most of their compounds (e.g., NaCl, CCl4, KI, AgBr).
However, they show positive oxidation states when they react with more electronegative elements (e.g., ClO2 with Cl +4, IF7 with I +7).
Rule 7: Monoatomic Ions: The oxidation state of a monoatomic ion is equal to the charge on it.
Rule 8: Sum of Oxidation Numbers: The sum of oxidation numbers of all the atoms in a compound or a polyatomic ion is equal to the charge on the compound or polyatomic ion.
By following these rules, we can systematically assign oxidation numbers to atoms in molecules and ions, aiding in understanding chemical reactions and balancing equations.
1) What is the oxidation state of oxygen in hydrogen peroxide?
A) +1
B) -2
c) +2
d) -1
Answer: D
Explanation: In peroxides, oxygen shows -1 oxidation state.