Question

What is the oxidation number of gold in the complex \[\rm{AuCl_4}^-\] ?
A. +4
B. +3
C. +2
D. +1

Answer 1

Hint:Oxidation number is the number of electrons that the atom of an element loses or gains during chemical bond formation. The oxidation number can be zero, negative, or positive. Use the oxidation number rules to calculate the oxidation number of gold.

Complete answer:
The complex given to us is \[\rm{AuCl_4}^-\].
Here we can see that the complex contains four chloride ligands and the overall charge on the complex is -1.
According to oxidation number rules, the oxidation number of chloride ions is -1.
The sum of the oxidation number of all atoms in a complex is equal to the charge on the complex. So, we can calculate the oxidation number of gold as follows:
(Number of gold atom) (Oxidation number of gold) + (Number of chlorine atom) (Oxidation number of chlorine) = charge on the complex
In the given complex there are 4 chlorine atoms and 1 gold atom present and the complex has charge -1.
So,
(1)(Oxidation number of gold) + 4(-1) = -1
The oxidation number of gold =+3
Thus, the oxidation number of gold \[{\rm{(Au)}}\] is +3.

Hence, the correct option is (B) +3.

Note:
Gold is a transition metal. Usually, transition metals have a variable oxidation state. So we can determine the oxidation number of transition metals in a complex using the oxidation number of ligands and charge on the complex. Oxidation state is the only apparent charge which represents the positive or negative character of the atom. It is necessary to denote the charge of oxidation state even if it is positive.