Question

What are the rule for oxidation number? ​

Answer 1

The oxidation number of a free element is always 0. The atoms in He and N2, for example, have oxidation numbers of 0. The oxidation number of a monatomic ion equals the charge of the ion. For example, the oxidation number of Na+ is +1; the oxidation number of N3- is -3.

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Answer 2

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Comparison between the electronegativities of two combining atoms is not an easy task in case of polyatomic molecules. That's why a set of rules has been given to calculate the oxidation number of an element in a given compound or ion. These rules are given below.

$<p style ="Color:cyan; font-family: cursive; background:black;font size25px;"> </p><p>Rule 1. The oxidation number of an atom in its free or clementary state or in any of its allotropes is zero. e.g, The oxidation of H in Hz. Sin Sg. Pin PO in Oz oro, C in diamond is zero.$

$<p style ="Color:pink; font-family: cursive; background:black;font size25px;"> </p><p>Rule 2. In case of ions having only one kind of atoms, the oxidation number of each atom is equal to charge present the ion. e.g. In case of</p><p>Na, Mg, Fe, Cl and 0²-, the oxidation states are respectively +1, +2, +3.-1-2.$

$<p style ="Color:red; font-family: cursive; background:black;font size25px;"> </p><p>Rule 3. The oxidation state of alkali metals in all of their compounds is always + 1. Similarly, in case of alkaline earth metals, it is always +2. For aluminium, oxidation state is always +3.$

$<p style ="Color:yellow; font-family: cursive; background:black;font size25px;"> </p><p>Rule 4. The oxidation state (OS) of oxygen in most of its compounds is -2, with an exception of peroxides and superoxides in which the oxidation state of oxygen is respectively - 1 and - 1/2. eg In H2O2 (Chydrogen peroxide), oxidation state of O is -1 (peroxide linkage is 0-0-) and in KO, (potassium superoxide), oxidation state of O is - 1/2Rule V. The oxidation state of hydrogen is generally +1 with an exception of metallic hydrides like Na, CaH2 etc. In these hydrides, oxidation state of hydrogen is - 1.$

$<p style ="Color:blue; font-family: cursive; background:black;font size25px;"> </p><p>Rule 5. The oxidation state of fluorine in all of its compounds is always - 1. Other halogens (i.e. chlorine, bromine and iodine) also exhibit - 1 oxidation state but it is not always true. In case of oxoacids and oxoanions, halogens (except fluorine) exhibit positive oxidation state.$

$<p style ="Color:orange; font-family: cursive; background:black;font size25px;"> </p><p>Rule 6. The algebraic sum of the oxidation numbers of all the atoms present in a compound must be equal to zero.$

$<p style ="Color:green; font-family: cursive; background:black;font size25px;"> </p><p>Rule 7. In case of polyatomic ions, the algebraic sum of oxidation number of all the atoms present in the ion must be equal to the charge on the ion. e.g. in case of carbonate ion (CO3-) it is equal to - 2.$