Question

How do you determine the oxidation number of Ca and Cl?

Answer 1

Hint: It is very important to understand the definition of oxidation number. To solve this question we have been aware of certain rules which are applied to calculate the oxidation number and position of above mentioned elements in periodic table.

Complete answer:
Oxidation number of an atom can be defined as the apparent charge on the atom in its compound.
There are certain rules that can be used to calculate the oxidation number of a particular atom:
1. The oxidation number of an element in its free state is zero.
Example: The oxidation number of ${{H}_{2}}$, $C{{l}_{2}}$,${{P}_{4}}$ are zero.
2. Oxidation number of an atom in a monatomic ion is equal to the number of positive or negative charges on the ion.
Example: Oxidation number of Al atoms in $A{{l}^{+3}}$ion is +3. The Cl atom in $C{{l}^{-}}$ion is -1.
3. The oxidation number of H in a compound is assigned as +1, except in metallic hydrides where its oxidation number is -1.
Example: The oxidation number of hydride is -1 in $Ca{{H}_{2}}$.
4. The oxidation number of O in compounds is assigned as -2.
Exceptions:
- In peroxides (${{H}_{2}}{{O}_{2}}$,$N{{a}_{2}}{{O}_{2}}$) where the oxidation number of O is -1.
-In superoxides ($K{{O}_{2}}$) the oxidation number of O is $-\frac{1}{2}$.
-The oxidation number of O in $O{{F}_{2}}$ is -1.
5.-The oxidation number halogen in halides or any compound is -1.
-The oxidation number of sulphur in sulphides is -2.
-The alkali metals that are the elements of group 1 possess oxidation number +1.
-The elements of group 2 that are alkaline metals possess oxidation number +2.
6. The algebraic sum of oxidation numbers of all atoms in a compound or ion is equal to the zero.
7. In coordination compounds,
- The oxidation number of neutral ligands like $NO$, $CO$,$N{{H}_{3}}$ and ${{H}_{2}}O$ is zero.
-The oxidation number of $C{{H}_{3}}$,${{C}_{6}}{{H}_{5}}$ are +1.
-The oxidation number of CN, OH, Cl are -1.
We know that Ca belongs to group 2 and Cl belongs to group 17 (halide)
From rule number 5 we can conclude that the oxidation number of Ca is +2 and the oxidation number of Cl is -1.

Note:
Oxidation number of H, O, the elements in free State, elements of certain groups such as group 1, group 2 and group 17 and certain organic compounds can be remembered easily by remembering the rules assigned to calculate oxidation state. This helps us to answer these kinds of questions within no time.