Oxidation state number can be used to determine the unknown element in these two compounds. They are used to determine how many electrons are given, taken or shared to form compounds.
Recall the elementary rules of oxidation numbers.
1. The sum of all oxidation numbers in a neutral compound is zero.
2. Chlorine, bromine, iodine have oxidation number of -1 ( except compounds with fluorine and oxygen)
Let oxidation number of element M be x.
Check rule 2. Chlorine has -1 oxidation number.
Now we write an equation of MCl₂ (neutral compound)
x + (2 * -1)= 0 ⇒ x₁= +2
For MCl₃
x + (3 * -1)= 0 ⇒ x₂= +3
So element has 2 different oxidation number in compounds, +2 and +3.
The element is iron (Fe) since it has +2 and +3 oxidation numbers in the compounds.
You need to learn it by hard. Unfortunately there is not an easier way to work out with these oxidation numbers.
The answer is iron (Fe).
