IMAT 2018 Q41 [Oxidation States]

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Compounds of chlorine include:

Cl_2O
KCl
KClO
KClO_3
KClO_4
Cl_2O_7

Which one of the following options correctly lists the oxidation states of chlorine that are shown in these compounds?

A. –1, +1, +5 and +7 only
B. –1, 0 and +1 only
C. –1 only
D. –1, 0, +1, +5 and +7 only
E. –1, +1, +3, +5 and +7 only

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To solve, find the oxidative states of the other molecules in the compound, sum them, and then Chlorine’s charge will be equal to the opposite charge of that number (i.e if other molecules summed charged = -1, then chlorine will have a charge of +1. Why? In neutral compounds, the total oxidative number must be equal to 0).

Cl_2O : Oxygen is a group 6 non metal, and will have an oxidative state of -2. Cl_2 - 2 = 0

Cl_2 = +2

Cl = +1

KCl : Potassium is a group 1 alkali metal, and will have an oxidative state of +1

Cl = -1

KClO : Potassium has an oxidative state of +1, oxygen has an oxidative state of -2.

1 + Cl - 2 = 0

Cl = +1

KClO_3 : Potassium has an oxidative state of +1, oxygen has an oxidative state of -2.

1 + Cl + (3)(-2) = 0

Cl - 5 = 0

Cl = 5

KClO_4 : Potassium has an oxidative state of +1, oxygen has an oxidative state of -2.

1 + Cl + (4)(-2) = 0

Cl - 7 = 0

Cl = +7

Cl_2O_7 : oxygen has an oxidative state of -2.

Cl_2 + (7)(-2) = 0

Cl = +7

\fcolorbox{red}{grey!30}{Therefore the answer is A, $-1, +1, +5, +7$}
3

I would also add, K would have +1 ON because after it looses 1 electron, it has complete octet, and becomes stable.

Chlorine would have the positive ON instead of Oxygen because it is less electronegative. Oxygen has EN values of 3.5 and Chlorine in 3.

And one more thing to note, ON of Chlorine will not go above +7 or below -1 because ON of +7 or -1 would mean it is like a noble gas and is stable.

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I don’t know why 0 is included in the answer…
Can an element in a molecule have an oxidation number of 0?
And as I know the answer is A.

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Please why is 0 included in the answer?

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Not really no. Diatomic molecules such as Cl2 are considered to have an oxidative state of 0 because each Cl has equal electron sharing but diatomic molecules arent really considered compounds

I also changed the answer because I had made a mistake, apologies and sorry for the confusion

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It was a typo, apologies!