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So to find the oxidation number of any element, look that element up in the periodic table and look to see what the easiest way it is for it to reach an oxidation that is "complete". 
So basically just look at the columns, 

Anythin in the first 3 columns will have to lose 1, 2, or 3 electrons: column 1 will have an oxidation of +1, column 2 will have +2, and column 3 will have +3. 

Columns 5,6,7: 5: elements in the 5th column will have an oxidation of -3, 6 column will have -2, 7 will have -1. 

Column 4: you are either + or - 4, it doesn't really matter, it's hard for them to give up 4 electrons and it's hard for them to gain 4. 

Column 8: they're already complete. They are the MOST reluctant to give up anything. 
H: is in the first column, which means it has 1 electron/proton. We say it has an oxidation of +1 because if it lost that 1, it would be complete. We dont' say 7 because it's easier for it to lose 1 than gain 7 to be complete (elements close to "completion" are more reluctant to give up their electrons). 

Cl: it's in the 7th column so it has an oxidation of -1. We say "-1" because it needs to GAIN one NEGATIVE electron (don't mistaken that as a "+1")! 
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Oxidation numbers are assigned to elements using these rules:Rule 1: The oxidation number of an element in its free (uncombined) state is zero — for example, Al(s) or Zn(s). This is also true for elements found in nature as diatomic (two-atom) elementsand for sulfur, found as:Rule 2: The oxidation number of a monatomic (one-atom) ion is the same as the charge on the ion, for example: Rule 3: The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic (many-atom) ion is equal to the charge on the ion. This rule often allows chemists to calculate the oxidation number of an atom that may have multiple oxidation states, if the other atoms in the ion have known oxidation numbers.Rule 4: The oxidation number of an alkali metal (IA family) in a compound is +1; the oxidation number of an alkaline earth metal (IIA family) in a compound is +2.Rule 5: The oxidation number of oxygen in a compound is usually –2. If, however, the oxygen is in a class of compounds called peroxides (for example, hydrogen peroxide), then the oxygen has an oxidation number of –1. If the oxygen is bonded to fluorine, the number is +1.Rule 6: The oxidation state of hydrogen in a compound is usually +1. If the hydrogen is part of abinary metal hydride (compound of hydrogen and some metal), then the oxidation state of hydrogen is –1.Rule 7: The oxidation number of fluorine is always –1. Chlorine, bromine, and iodine usually have an oxidation number of –1, unless they’re in combination with an oxygen or fluorine
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