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Oxidation Numbers Watch

1. Hi can somebody please help me with oxidation numbers. I am retaking OCR as chemistry atoms,bonds and groups and really struggling with oxidation numbers!!

I have these questions which say...

Work out the oxidation state of the element in bold.
e.g
NaClO3.
All it says in my text book is

Uncombined element = 0
Combined Oxygen= -2
Combined Hydrogen = +1
Simple Ion = Charge on ion
Combined Fluorine = -1.

Using this i can only work out the 03 part which is -6. But how am I supposed to work out the oxidation numbers of the Na and Cl. The answer says Cl = +5.

Would really appreicate the help.
2. Na is in group 1, so take it as being plus 1

if it asked you for Na, you would take Cl as being -1
3. Generally [but not always the case] the rule is:

group 1 plus 1
group 2 plus 2
group 3 plus 3
group 4 0
group 5 -3
group 6 -2
group 7 -1

-6 + 1 = -5

x + -5= 0
x=5
4. Thankyou
That has really helped, I can now do all the questions!
5. (Original post by Alwee)
Thankyou
That has really helped, I can now do all the questions!
6. (Original post by nazirard)
Generally [but not always the case] the rule is:

group 1 plus 1
group 2 plus 2
group 3 plus 3
These are good to go...

(Original post by nazirard)

group 4 0 (never the case except in the element itself)
group 5 -3
group 6 -2
group 7 -1
These are not...

The 'rules' below are better
--------------------------

Hydrogen is always +1 unless bonded to a metal in a binary salt (i.e. a metal hydride) in which case it's -1

Oxygen is always -2 unless a peroxide,in which case -1

The binary salts with metals have:

group 5 -3
group 6 -2
group 7 -1

BUT otherwise you have to add up.

A compound has a sum of zero and an ion has a sum of the charge on the ion.

Na2SO4

Na (group 1) = +1 , therefore 2 x Na = +2
Oxygen = -2 therefore 4 x O = -8
---------------------------------------------sum
total = -6, therefore sulphur must = +6
7. (Original post by charco)
These are good to go...

These are not...

The 'rules' below are better
--------------------------

Hydrogen is always +1 unless bonded to a metal in a binary salt (i.e. a metal hydride) in which case it's -1

Oxygen is always -2 unless a peroxide,in which case -1

The binary salts with metals have:

group 5 -3
group 6 -2
group 7 -1

BUT otherwise you have to add up.

A compound has a sum of zero and an ion has a sum of the charge on the ion.

Na2SO4

Na (group 1) = +1 , therefore 2 x Na = +2
Oxygen = -2 therefore 4 x O = -8
---------------------------------------------sum
total = -6, therefore sulphur must = +6
Some of what you said i presumed he already knew as it's in the ocr book (the situation with ions).

In the AS spec it says you don't need to know about the oxygen and peroxide situation.

My chem teacher told us this is the best way to know it at AS level.

But thanks for the corrections.
8. This is the list of priority:

Group 1 is always +1
Group 2 is always +2
Group 3 is +3
F is -1
H is +1
O is -2
Cl is -1
You can work out others from this as you tend to have compounds given with at least one of these elements in.

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Updated: December 16, 2010
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