Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Find out the charge of a compound/ion

  1. Sep 12, 2004 #1
    How do you find out the charge of a compound/ion when a question just says "Calcium ion"? Do I need more info on it, am I missing something?
    I'll give a few examples:
    Sulfite ion
    sulfur dioxide
    sulfur trioxide

    I need to know the number that goes in front of the charges. Please don't just give me an answer for these 3, if you could also explain how you got the answer it would greatly be appreciated. Thanx.
    Last edited by a moderator: Mar 7, 2013
  2. jcsd
  3. Sep 12, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    First you need to know the most common oxidation states of several different elements. These are easy to remember if you know the groups that these elements belong to in the Periodic Table. Group I has Li, Na, K (the others are less important), and all these have the oxidation state of +1. Group II has Be, Mg, Ca and these have oxidation state +2. The transition metals, from Sc to Zn and below have different oxidation states that are useful to remember for the common elements. In group III, B, Al, Ga have ox. number +3. Gr. IV has C, Si, Ge, Sn, which have ox. no. +4. From Gr V on the oxidation numbers are negative (ox. no. = Gr. no. - 8). So Gr. V (N, P) has ox. no. -3. Gr. VI (O, S) has ox. no. -2. However, N is often considered +5, but sometime +3 and S can be either +6 or +4 (which one it is depends on the compond, and can be determined from the other elements in that compound). Gr. VII (F, Cl, Br) is always -1. And Gr. VIII has ox. no. 0. So, elements of Gr. VIII never form compounds.

    To find the oxidation no. of a radical (like sulfite, nitrate, etc.) you simply add the oxidation states of the different elements in it.

    Sulfite is SO3(2-). We know that each O is -2, so the 3 O's make up -6. So, SO3(2-) has a total charge of -2. Let the oxidation state of S be x. Then x + 3(-2) = -2. Or x-6=-2. Hence, x=4. The ox. no. of S is +4 in this case.

    In a compound (or stable element), the total charge is always 0. So, the sum of positives must equal the sum of the negatives.

    So, in sulfur dioxide (SO2) , the total ox. no. is 0. We know for sure that O is -2. And there are 2 O's, so the total negative charge is -4. So, for the compound to have zero total charge, S must be +4.

    What would be the ox. no. of S in sulfur trioxide (SO3) ?
  4. Sep 12, 2004 #3


    User Avatar
    Science Advisor
    Homework Helper

    Your questions pertains to formal charges, and you should find this topic in your book and study it. As for elements, ways to find the common charges that elements employ is to observe which group they are in.
  5. Sep 13, 2004 #4
    Thanx, for the help, but I just asked my teacher. There are ceratain compounds and elements that we just need to remember. The charges are given in the book, but thanx anyway, i'm sure ill have more questions soon.
  6. Sep 13, 2004 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    You need to remember the elements, the compounds can be figured out. For many of the elements, knowing the position in the periodic table helps with the memorizing.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook