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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 Sulfate 3.      Na2SO4
4.Copper (II) Chloride 4.      CuCl2
5.Barium Nitrate 5.      Ba(NO3)2
6.Aluminum Hydroxide 6.      Al(OH)3
7.Mercury (II) Phosphate 7.      Hg3(PO4)2
8.Aluminum Silicate 8.      Al2(SiO3)3
9. Copper (II) Bromide 9.      CuBr2
10.Lead (II) Chlorite 10.     Pb(ClO2)2
11.Silver Cyanide 11.     AgCN
12.Ammonium Oxide 12.     (NH4)2O
13.Aluminum Perchlorate 13.     Al(ClO4)3
14.Tin (II) Chloride 14.     SnCl2
15.Nickel (III) Acetate 15.     Ni(C2H3O2)3
16.Potassium Sulfide 16.     K2S
17.Magnesium Bisulfate 17.     Mg(HSO4)2
18.Iron (II) Phosphate 18.     Fe3(PO4)2
19.Cobalt (II) Hydrogen Sulfate 19.     Co(HSO4)2
20.Chromium (II) Bicarbonate 20.     Cr (HCO3)2
21.Sodium Hypochlorite 21.     NaClO
22.Barium Carbonate 22.     BaCO3
23.Zinc (II) Permanganate 23.     Zn(MnO4)2
Formula Name
AlPO4 aluminum phosphate
KNO2 potassium nitrite
NaHCO3 sodium hydrogen carbonate
CaCO3 calcium carbonate
Mg(OH)2 magnesium hydroxide
Na2CrO4 sodium chromate
Ba(CN)2 barium cyanide
K2SO4 potassium sulfate
NaH2PO4 sodium dihydrogen phosphate
NH4NO3 ammonium nitrate
Sn(NO3)2 tin(II) nitrate
FePO4 iron(III) phosphate
Cu2SO4 copper(I) sulfate
Ni(C2H3O2)2 nickel(II) acetate
HgCO3 mercury(II) carbonate
Pb(OH)4 lead(IV) hydroxide
Cu2Cr2O7 copper(I) dichromate
Cu(ClO3)2 copper(II) chlorate
FeSO4 iron(II) sulfate
Hg2(ClO4)2 mercury(I) perchlorate
KClO3 potassium chlorate
SnSO4 tin(II) sulfate
Al(MnO4)3 aluminum permanganate
Pb(NO3)2 lead(II) nitrate
Mg3(PO4)2 magnesium phosphate
CuH2PO4 copper(I) dihydrogen phosphate
CaHPO4 calcium hydrogen phosphate
Fe(HCO3)3 iron(III) hydrogen carbonate
Na2CO3 sodium carbonate
MnSO4 manganese(II) sulfate
Ca(ClO3)2 calcium chlorate
Fe(OH)3 iron (III) hydroxide
Cu2SO4 copper (I) sulfate
KMnO4 potassium permanganate
NaOH sodium hydroxide
Fe(NO3)2 iron (II) nitrate

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