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Chemical Formula Writing

 

 

The procedure that can be followed when confronted with the name of a compound and you wish to write its formula is as follows:

  1. Identify the symbol of the cation (first part of the name) and the anion

  2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol

    Cations (Positive Ions)

    All Group 1 elements in the Periodic Table are +1 in compounds.

    All Group 2 elements in the Periodic Table are +2 in compounds.

    Transition elements (have a few charges) will have a Roman Numeral to tell you what positive charge to use.

    silver is 1+, Zinc is 2+ and Aluminum is 3+

    Polyatomic Ions -just use its charge.

     

    Anions  (Negative Ions) 

    Group 17 are 1-  (will end with -ide)

    Group 16 are 2-  (will end with -ide)

    Group 15 are 3-   (will end with -ide)

    Polyatomic Ions -just use its charge.  (Generally ends with -ate or -ite, watch out for hydroxide  OH- and cyanide CN-)  

     

  3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero.

   4.  Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.

So for example what is the formula of Copper (I) Oxide?

  1. Identify the symbols of the cation and anion

    Copper is Cu and Oxide is O

  2. Identify the charge for each and place above the symbol in parenthesis

    For Copper I that would be 1+ and for Oxide that would be 2-

  3. Balance the positive and negative charges

    Since each Copper is 1+ and each Oxide is 2- then it will take two Cu+ to balance one oxide with a 2- so that 

    2(1+) + 1(2-) = 0. The numbers outside the parenthesis become the subscripts in the formula

  4. Write the formula placing the subscripts right after the symbol they go with.

    Cu2O

Notice that we don't bother to place a subscript 1 after the Oxide symbol. That is because a subscript one is understood to be so. If it was zero it wouldn't appear at all in the formula.

Also note that all binary compounds end in "ide"

Let's try another:

What is the formula of Calcium Nitride?

  1. Identify the symbols of each part of the name

    Calcium symbol is Ca and Nitride symbol is N

     

  2. Identify the charge for each

    Calcium belongs from Group 2 which always has a +2 and Nitride will be a single Nitrogen with a -3 charge

     

  3. Balance charge

    Since Calcium is +2 and Nitride is -3 the only way to balance them is to have three Calcium's and two nitrides

     

  4. Write the symbol beginning with the symbol that is first in the name and include the subscript after each symbol

    Ca3N2

    Formula writing with Polyatomic Ions

    1. Identify the symbol of the cation (first part of the name) and the anion

    The symbol for Iron is Fe and the symbol for Carbonate which is a polyatomic ion is CO3

    2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol

    The valence for Iron (III) is 3+ and the valence for Carbonate is 2-

    3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero. Since an Iron (III) has a +3 charge and the Carbonate ion has a 2- then it would take two Fe3+ units to balance three CO32- units

    4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.

    Fe2(CO3)3

    Try These Ionic Compounds Highlight to reveal the formula
    1.Magnesium Nitride 1.      Mg3N2
    2.Iron (III) Oxide 2.      Fe2O3
    3.Sodium Sulfate3.      Na2SO4
    4.Copper (II) Chloride 4.      CuCl2
    5.Barium Nitrate 5.      Ba(NO3)2
    6.Aluminum Hydroxide6.      Al(OH)3
    7.Mercury (II) Phosphate7.      Hg3(PO4)2
    8.Aluminum Silicate8.      Al2(SiO3)3
    9. Copper (II) Bromide9.      CuBr2
    10.Lead (II) Chlorite10.     Pb(ClO2)2
    11.Silver Cyanide11.     AgCN
    12.Ammonium Oxide12.     (NH4)2O
    13.Aluminum Perchlorate13.     Al(ClO4)3
    14.Tin (II) Chloride14.     SnCl2
    15.Nickel (III) Acetate15.     Ni(C2H3O2)3
    16.Potassium Sulfide16.     K2S
    17.Magnesium Bisulfate17.     Mg(HSO4)2
    18.Iron (II) Phosphate18.     Fe3(PO4)2
    19.Cobalt (II) Hydrogen Sulfate19.     Co(HSO4)2
    20.Chromium (II) Bicarbonate20.     Cr (HCO3)2
    21.Sodium Hypochlorite21.     NaClO
    22.Barium Carbonate22.     BaCO3
    23.Zinc (II) Permanganate23.     Zn(MnO4)2

     

    FormulaName
    AlPO4aluminum phosphate
    KNO2potassium nitrite
    NaHCO3sodium hydrogen carbonate
    CaCO3calcium carbonate
    Mg(OH)2magnesium hydroxide
    Na2CrO4sodium chromate
    Ba(CN)2barium cyanide
    K2SO4potassium sulfate
    NaH2PO4sodium dihydrogen phosphate
    NH4NO3ammonium nitrate
    Sn(NO3)2tin(II) nitrate
    FePO4iron(III) phosphate
    Cu2SO4copper(I) sulfate
    Ni(C2H3O2)2nickel(II) acetate
    HgCO3mercury(II) carbonate
    Pb(OH)4lead(IV) hydroxide
    Cu2Cr2O7copper(I) dichromate
    Cu(ClO3)2copper(II) chlorate
    FeSO4iron(II) sulfate
    Hg2(ClO4)2mercury(I) perchlorate
    KClO3potassium chlorate
    SnSO4tin(II) sulfate
    Al(MnO4)3aluminum permanganate
    Pb(NO3)2lead(II) nitrate
    Mg3(PO4)2magnesium phosphate
    CuH2PO4copper(I) dihydrogen phosphate
    CaHPO4calcium hydrogen phosphate
    Fe(HCO3)3iron(III) hydrogen carbonate
    Na2CO3sodium carbonate
    MnSO4manganese(II) sulfate
    Ca(ClO3)2calcium chlorate
    Fe(OH)3iron (III) hydroxide
    Cu2SO4copper (I) sulfate
    KMnO4potassium permanganate
    NaOHsodium hydroxide
    Fe(NO3)2iron (II) nitrate

     

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