Question

How to calculate oxidation number??

Answer 1

Well theres the method of ( let x be an element and y be its number of valance electron, this is EXCLUDING TRANSITION METAL) so X has , 4y from group 4a. Its oxidation would be -4. Group 1 and 2 (alkaline and alkali earth metal) are all positive oxidation ( like sodium , calcium, etc...). They will be mostly found in salt and will always create positive Ion ( maybe except for Hydrogen which is an exception). Afterwards, group 3A can go either way depending on the reaction, afterwards group 4A-7A ( so halogens, etc...) will all form Anions. Hope I helped

Answer 2

There are some basic rules that will often work for most of general chemistry. The first are based on groups and elements that usually have the same oxidation state:

1) Group 1A metals are always +1.
2) Group 2A metals are always +2.
3) Group 7A halogens are always -1.
4) Hydrogen is +1 in compounds with nonmetals, -1 in compounds with metals.
5) Oxygen is -2 except in peroxides, where it is -1.
6) Any element in its standard state is 0.
7) The sum of all the oxidation states must equal the charge on each formula unit, or molecule.

That will get you through a lot, e.g.:

CO2: oxygen -2 (rule 5), carbon +4 (rule 7).
H2O: oxygen -2 (rule 5), hydrogen +1 (rule 4).
NH3: hydrogen +1 (rule 4), nitrogen -3 (rule 7).
NO3-: oxygen -2 (rule 5), nitrogen +5 (rule 7).
NaCl: sodium +1 (rule 1), chlorine -1 (rule 3).
MgOH: magnesium +1 (rule 1), oxygen -2 (rule 5), hydrogen +1 (rule 4). Note that H is part of the OH- molecular anion here.
Fe2O3: oxygen -2 (rule 5), iron +3 (rule 7).
FeO: oxygen -2 (rule 5), iron +2 (rule 7).
KMnO4: potassium +1 (rule 1), oxygen -2 (rule 5), manganese +7 (rule 7).

Things can get stickier when you wander off into compounds involving Period 4 and higher elements, particularly the transition metals, which can have many oxidation states. It helps to remember that Period 2 and Groups 1A, 2A and 7A tend to be more consistent. So for BrF5 we guess fluorine sticks with -1, being in Period 2, and Br goes to +5 by rule 7. Similarly for SO2 we assume oxygen, beingin Period 2, sticks with -2, which makes the oxidation state of sulfur +4. An exception to this is nitrogen, which is unusual in being the Period 2 element with the most possible oxidation states. (-3 in Li3N, -2 in NH2NH2, -1 in N2H2, 0 in N2, +2 in NO, +3 in HNO2, +4 in NO2, and +5 in HNO3). A remarkably versatile element! The only competitor is probably manganese, a transition metal, which is observed in oxidation states from 0 to +7, inclusive.

But usually by the time you start pondering more complex compounds, oxidation state becomes something to observe and explain, rather than predict ab initio.

Answer 3

there are some rules to find oxidation. Of which element you wanna find oxidation number?

Answer 4

remember every compound has 0 or netral. :)