Video Transcript
Give the oxidation state of
chlorine in ClO−.
The question asks us to determine
the oxidation state of chlorine in the hypochlorite anion whose chemical formula is
ClO−. Let’s first recall the meaning of
the term oxidation state and then explain the oxidation state of elements in an
ionic compound and then in a covalent compound with a charge and determine the
answer to this question.
We use the term oxidation state to
describe the number of electrons lost or gained by an atom of an element. An atom in its elemental form —
that is, an atom that is not bonded to another element — is given a zero oxidation
state value.
In an ionic compound, a positive
oxidation state is assigned to atoms which have lost electrons and thus formed
positively charged ions of an ionic compound, which we call cations. Chemical elements in a compound are
given a negative oxidation state when they gain electrons to form negatively charged
ions called anions.
Let’s look at the ions of sodium
chloride as an example of how to assign oxidation states to an ionic compound. We know that atoms in their
elemental state have a zero oxidation state. Both the sodium and chlorine
reactants are in their elemental form. We know the sodium chloride product
is ionic. It is composed of one negatively
charged chloride ion for each positively charged sodium ion. The sodium ions have a plus-one
oxidation state as sodium atoms lose one electron when forming the sodium
cation. Similarly, we can see that the
chloride ions have a minus-one oxidation state as chlorine atoms gain an electron
when forming a chloride anion.
In this second example, we see the
ionic compound Na2O, sodium oxide, forming from its elements sodium and oxygen. The reactants are in their
elemental form, and thus both have a zero oxidation state. The product is made of sodium one
plus ions. We give the sodium ions a plus-one
oxidation state as sodium atoms lose an electron when forming the Na+ cation. The oxygen ions have a charge of
two minus, and we assign an oxidation state of minus two because each oxygen atom
gains two electrons as it turns into the O2− anion.
So far, we have looked at how to
assign an oxidation state to ionic compounds. Notice that hypochlorite, given in
the question, is an ion. However, within this anion, the
chlorine atom and the oxygen atom are bonded covalently. Covalently bonded compounds, or
ions which contain a covalent bond, such as the hypochlorite ion, are slightly
different because the component atoms do not gain or lose electrons. Instead, the bonding electrons are
shared but are not distributed equally across the covalent bond.
We assign a positive oxidation
state for the atom in a covalent bond with a lower electronegativity number. And we assign a negative oxidation
state to the atom in a covalent bond which has a higher electronegativity
number. So, let’s take our ion in question,
the hypochlorite ion, which has a covalent bond joining its two atoms.
The Pauling electronegativity value
of chlorine is 3.16, and oxygen’s value is 3.44. Oxygen has a higher
electronegativity value. Therefore, it is assigned a
negative oxidation state. So we can write a negative sign
above the oxygen atom.
Since the bonding electrons are
unevenly distributed, and since oxygen is more electronegative, we can treat the
oxygen atom as if it were ionic. We can thus see that the number
associated with the negative sign must be two. We write the number after the sign
for oxidation numbers. Chlorine has the lower
electronegativity value. Therefore, it is assigned a
positive oxidation state. We write the positive sign above
the chlorine atom.
Since the overall charge of the
hypochlorite ion is one minus, we can conclude that the oxidation state of chlorine
in this ion must be plus one. The oxidation state of the oxygen
atom is minus two, and so the oxidation state of the chlorine atom must be plus
one.
Finally, give the oxidation state
of chlorine in ClO−. The answer is plus one.
