BackOxidation Numbers and Their Rules in GOB Chemistry
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Oxidation Numbers in Chemistry
Definition and Importance
The oxidation number (also called oxidation state) of an element is a value that represents the number of electrons lost or gained by an atom in a compound, compared to its elemental state. Oxidation numbers are essential for understanding redox reactions, electron transfer, and chemical bonding.
Oxidation number: An element's ability to lose, gain, or share electrons when alone or in a compound.
In its natural state (standard state), an element's oxidation number is always zero.
Examples of Oxidation Number Zero
Elements in their natural state, such as Na, Cl, He, and Mn, have an oxidation number of zero.
Example: Which of the following compounds would have an oxidation number or oxidation state equal to zero?
a) Na
b) Cl
c) He
d) Mn
Oxidation Numbers in Ions
Relationship to Ionic Charge
For ions, the oxidation number is equal to the charge of the ion. For a monatomic ion, this is straightforward:
Oxidation number of a monatomic ion = ionic charge
Example: Which of the following elements would have the most positive oxidation number based on its ionic form?
a) Silver, Ag
b) Scandium, Sc
c) Sodium, Na
d) Sulfur, S
Practice: Which of the following elements would have the lowest oxidation number?
a) Indium, In
b) Strontium, Sr
c) Argon, Ar
d) Manganese, Mn
Oxidation Number Rules
General Guidelines
Oxidation numbers do not always correspond to real charges, especially in covalent compounds. Therefore, a set of rules is used to assign oxidation numbers:
When different elements are in a compound, specific rules are used to calculate oxidation numbers.
Specific Oxidation Number Rules
Element(s) | Oxidation Number | Notes/Examples |
|---|---|---|
Group 1A | +1 | When connected to any other element |
Group 2A | +2 | When connected to any other element |
Fluorine | -1 | When connected to any other element |
Hydrogen | +1 | When connected to nonmetals (e.g., HCl, H2O, NH2) |
Hydrogen | -1 | When connected to metals (e.g., NaH, CaH2, BH3) |
Oxygen | -2 | When it exists as a peroxide or superoxide, it can be -1 or -1/2 (e.g., H2O2, HO2) |
Oxygen | -1 | In peroxides (e.g., H2O2, Na2O2) |
Oxygen | +2 | In compounds with fluorine (e.g., OF2) |
Group 7A (other than F) | -1 | When connected to metals or nonmetals (except O or other halogens) |
Example: Which compound has oxygen with the lowest oxidation state?
a) NaO2
b) CO
c) Cs2O
d) O2
Oxidation Number of Non-listed Elements
Steps to Determine Oxidation Number
When asked to determine the oxidation number of a non-listed element within a compound, follow these steps:
Step 1: Treat the non-listed element as x.
Step 2: Use the list to write the known oxidation number of the other elements.
Step 3: If an element has a subscript, remember to distribute it.
Step 4: Add up the oxidation numbers, create an equation, and make it equal to the charge of the compound.
Example: What is the oxidation number of sulfur in SO2?
Formula for Calculating Oxidation Number
For a neutral compound:
For a polyatomic ion:
Additional info: These rules and examples are foundational for understanding redox reactions, balancing equations, and predicting chemical behavior in GOB Chemistry.
