element numbers question?

Why is potassium oxide K2O instead of KO what does that 2 mean.


And what about aluminum sulphide?. Al2S3

1 Answer

  • 7 years ago
    Favourite answer

    First you need to know about electrovalency, ionic compounds and the names and charges of some cations (positive ions) and anions (negative anions).

    An ion is a charged atom.

    Anyway potassium oxide is an ionic compound made up of a potassium cation (K ) and an oxide anion (O2-). Always the ions combine so that both charges balance, making the compound electrically neutral (no chrage). So 2 K ions must join with 1 O2- ion to balnce the charges. Therefore the 2 charges blance the 2-.

    Therefore the compound is K2O. The 2 indicates that there are 2 potassium ions in the compound. The formula is not KO as the charges will not balance.

    The easiest way to find out this is to interchange the charge numbers with the atoms. K will go to O and O2- will go to K. Remeber the numbers are interchanged and not the charges.

    So it becomes K2O.

    Aluminium sulphide

    Al3 and S2-

    Al3 to S and S2- to Al

    S3 and Al2

    Write the compound correctly: aluminium before sulphide:

    Therefore the formula is Al2S3. Now the charges balance: There are 6 positive charges and 6 negative charges. This method is only for electrovalent or ionic compounds. Cheers!

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