Question

# The oxidation state of Cl in ${\rm{CaOC}}{{\rm{l}}_{\rm{2}}}$ is: A ) -1and +1B ) +2C ) -2D ) none of these

Hint: Oxidation number is the total number of electrons an atom gains or loses to form a chemical bond with another atom. Thus, if a calcium atom loses two electrons, the oxidation number of calcium will be +2. Similarly, if an oxygen atom gains two electrons, the oxidation number of oxygen will be -2. The sum of the oxidation states of all the atoms / ions neutral compounds is zero.

Oxidation state is also known as the oxidation number. It is the total number of electrons either gained or lost by an atom to form a chemical bond with another atom.
Chlorine usually forms compounds in which it has oxidation states ranging from-1 to +7. Thus, in compounds such as sodium chloride and hydrogen chloride, the oxidation state of chlorine is -1 as it gains 1 electron during compound formation. On the other hand, in compounds such as ${\rm{C}}{{\rm{l}}_2}{{\rm{O}}_7} {\rm{\; and\; KCl}}{{\rm{O}}_4}$, the oxidation state of chlorine is +7.
Bleaching powder is ${\rm{CaOC}}{{\rm{l}}_{\rm{2}}}$.
Let X be the oxidation state of chlorine in ${\rm{CaOC}}{{\rm{l}}_{\rm{2}}}$. The oxidation states of calcium and oxygen are +2 and -2 respectively. In a neutral molecule, the sum of the oxidation states of all the atoms / ions is zero.
Calculate the oxidation state of iron in ${\rm{CaOC}}{{\rm{l}}_{\rm{2}}}$.
(+ 2) + (- 2) + 2( x ) = 0
2x = 0
x = 0
The oxidation state of Cl appears to be zero. However, this is not true.
${\rm{CaOC}}{{\rm{l}}_{\rm{2}}}$ can also be written as ${\rm{Cl}} - {\rm{Ca}} - {\rm{OCl}}$.
Two types of Cl atoms are present. One Cl atom is directly attached to Ca atom. This Cl atom has an oxidation state of -1 as calcium is electropositive alkaline earth metal with oxidation state of +2. Other Cl atom is directly attached to the O atom. This Cl atom has an oxidation state of +1 as oxygen is a highly electronegative element.

Hence, the option A ) -1 and +1 is the correct answer.

Note: Do not give the answer that both Cl atoms have 0 oxidation state. The chlorine atom, that is directly attached to the calcium atom, will gain one electron from the calcium atom, and will show the oxidation state of -1. Here, chlorine gains one electron from calcium because chlorine is more electronegative than calcium. On the other hand, the chlorine atom, that is directly attached to oxygen atom, will lose one electron to oxygen atom, and will show the oxidation state of +1. Here, chlorine loses one electron to oxygen because chlorine is less electronegative than oxygen.