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Thread: Valency states

  1. #1 Valency states 
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    I cant get any good help on this and would appreciate anyones assistance and time in this problem. HOW do you calculate valecy states of elements? PLEASE if its not possible tell me ill find a way to go on living if not it would be great to have any input on the matter, chem is a bitch and this would help profoundly!


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  3. #2  
    Moderator Moderator AlexP's Avatar
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    Could you explain in more detail what you're after? Do you want to know in general how to find the charges that certain atoms will tend to take on when they bond?


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  4. #3  
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    If what you're after is the 'oxidation state', this is the general method, although there are shortcuts for simpler cases.

    Suppose you want to calculate the oxidation state of all atoms in potassium thiocyanate (KNCS).

    Write down all the atoms separately:

    K N C S

    Count how many electrons you have in total in this compound. You get that by adding up the valence shell electrons of each atom (which, for non-transition elements is the group number in the periodic table). So 1 for potassium, 5 for nitrogen, 4 for carbon and 6 for sulphur. 1+5+4+6 = 16.

    Lookup the electronegativities of the elements and sort them decreasingly (N>S>C>K).

    Now 'add' the electrons to each atom in order of decreasing electronegativity. When you get 8 electrons around an atom, go to the next, and proceed until you 'finish' the electrons. I will write the number of electrons in parenthesis next to each atom in the example:

    K(0) N(8) C(0) S(8)

    To obtain the oxidation state, subtract the numbers obtained here from the original number of valence shell electrons:

    K(1-0) N(5-8) C(4-0) S(6-8)

    that is:

    K(+1) N(-3) C(+4) S(-2)

    You may note that +1-3+4-2=0, because the overall compound is electrically neutral. If you did the same with an ion (e.g. NCS- or NH4+) you would get the corresponding charge as the sum.
    For atoms with the same electronegativity, add the electrons one by one to each alternatively.
    And of course you may get less than 8 electrons around an atom.

    Try it out with some common compounds such as H2O, NH3, HN3, H2SO4, CaCO3, K3Fe(CN)6... you'll see it works.

    Just note that the oxidation state or valence has a theoretical value most of the time; it is not the actual electrical charge on the atom.

    And please don't say chemistry is a bitch. Chemistry is a very interesting subject, often badly taught, often difficult to understand, but as an abstract concept it can't possibly have anything against you personally.
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