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Working out chemical formulae

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    I'm studying the OCR 21st Century separate sciences.
    My questions is relating to the formula CaCO3.

    I understand that calcium has a 2- charge, and therefore assume CO3 has a 2- charge. However, I don't know how to work out that CO3 has a 2- charge.

    Am I meant to be able to calculate this, or just remember it as a fact?

    I haven't taught C21 for a while, but when I did, you weren't meant to be able to remember that CO3 has a 2- charge, but you were meant to be able to work out the charge if you were told a compound's formula and the other ion's charge - exactly as you did.

    e.g. if sodium carbonate is Na2CO3 and Na is 1+, then what is the charge on the CO3 ion?

    It is important to note that when I say charge, I mean oxidation state. At this level (GCSE) it is sufficient and perfectly ok to think of it as charge but it isn't fully correct. So when I say charge in my post following I really mean oxidation state - but at GCSE you do t need I worry about it.
    Oxygen is in group 6 so has a charge of 2- . You may wonder why it isn't 6+ . This is because oxygen is a strong oxidising agent - meaning it takes electrons from things - so oxygen itself is reduced, it gains electrons form things.
    So O2 will have a charge of 4- .O3 will have a charge of 3- . Carbon is in group 4 and so has a charge of +4. Again you may wonder why this isn't -4. This is because carbon is usually a reduced agent so it loses electrons to things therefore becoming more positive.
    So carbon is +4 and O3 is -6 so the overall charge is 2- .
    I would do this for all the main ions, such as sulphate, phosphate, nitrate etc.
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