HOW TO WRITE CHEMICAL FORMULAS

DIRECTIONS: If the answer to a question is No, go to the next similarly numbered or lettered question. If the answer is Yes, follow any directions listed and go on to the first subdivision. Once you have said Yes to a subdivision, all following subdivisions of that question are skipped unless they are preceded by an *. In this manner continue through questions 1 ---> 5 until the ions for the formula have been written. Then skip to number 6 and continue until the complete formula has been written.
  1. Does the name of the compound include the word acid ?
    Write H (oxidation number +1) as the first element in the compound.
    1. Does the name of the compound have the prefix hydro-?
      This means that the acid does not contain oxygen; therefore, the second part of the formula is either a monatomic ion (single element) or a polyatomic ion which does not contain oxygen (i.e. cyanide).
      Drop the prefix "hydro-", change the "-ic" ending to "-ide" and write this ion as the second part of the formula.
      [Go to #6]
    2. Does the first part of the acid name end in -ic ?
      Change the "-ic" to "-ate" and write this polyatomic ion as the second part of the formula.
      [Go to #6]
    3. Does the first part of the acid name end in -ous ?
      Change the -ous to -ite and write this polyatomic ion as the second part of the formula.
      [Go to #6]
  2. Does the name of the compound have more than two words?
    1. Does the last word end in -hydrate ?
      Write the formula for the compound listed before the word ending in -hydrate. (To do this continue on with section 2 (b) after you finish reading this section.) Follow this formula with a dot . , then the number for the Greek prefix attached to the word hydrate, ending with the formula for water.
      (Example: Copper (II) sulfate pentahydrate CuSO4 . 5 H2O)
    2. *Does the name of the compound have the word hydrogen in the middle of the name?
      This indicates an acid salt. To write the formula and oxidation number for the negative polyatomic ion of an acid salt:
      1. Write the formula for the polyatomic ion listed after the word hydrogen.
      2. Write an H in front of the formula for this polyatomic ion. If the word hydrogen in the compound name contains a Greek prefix, write the number for the Greek prefix (if greater than 1) as the subscript for the Hyou just wrote. No Greek prefix indicates that there is only one possible acid salt for this polyatomic ion; and therefore, no subscript is required. (Remember a subscript one is understood and not written in compound formulas.)
      3. To determine the oxidation for the negative polyatomic ion of an acid salt, add +1 for each hydrogen you added to the oxidation number of the original ion. (For example: PO43- phosphate; H2PO41- dihydrogen phosphate) Now use this acid salt polyatomic ion as you would any other polyatomic ion in writing the formula of the compound listed.

  3. Is the first word of the compound name an element or root of an element?
    Write the symbol for this element as the first part of the compound formula.
    1. Is the first element a nonmetal?
      For a binary compound containing two nonmetals, use the Greek prefixes before the names of the elements to determine the number of atoms of each in the compound. If there is no Greek prefix for the first element, there is just one atom of that element in the formula.

      Greek Prefixes Number
      mono- 1
      di- 2
      tri- 3
      tetra- 4
      penta- 5
      hexa- 6
      hepta- 7
      octa- 8
      nona- 9
      deca- 10

      Note: If the name of the element begins with a vowel, the last vowel of the prefix is sometimes omitted.
      Complete entire formula using Greek prefixes.
      [STOP]
    2. Does the first word in the name of the compound end in -ous or -ic ?
      This means the metal ion (positive ion) in this compound has a varying oxidation number. [See Table of Common Cations with Varying Oxidation Numbers] For the -ous ending use the lower of the two oxidation numbers; for the -ic ending use the higher oxidation number. (Arsenic is an element with a name which ends in -ic. If the word arsenic in the compound name is followed by (III) , this means that it has an oxidation number of +3 and not +5 as the -ic ending alone would indicate.)
    3. Is there a Roman numeral in parentheses in the middle of the compound name?
      This indicates a varying oxidation number for the first ion in the compound formula. Use the value of the Roman numeral as a positive oxidation number for the first element listed in the compound name.
    4. Find the element on the Periodic Table. Locate the Roman numeral at the top of the column containing this element. Use the value of this Roman numeral as a positive oxidation number for the element. [Or see Table of Oxidation Numbers for Common Cations .]

  4. Does the name of the compound end in -ide ?
    1. Is the word ending in -ide the name of a polyatomic ion?
      Hydroxide OH1- Ferrocyanide Fe(CN)64-
      Cyanide CN1- Ferricyanide Fe(CN)63-

      If so, write the formula for this ion as the second part of the formula for the compound.
    2. If the word ending in -ide is not a polyatomic ion, this means that the second part of the compound formula is a single element.
      Write the symbol for this element with its oxidation number. The oxidation number for the element written as the second part of the compound is determined by subtracting 8 from the Roman numeral located at the top of the column containing this nonmetal on the Periodic Table. Note: The oxidation number is negative. [Or see Table of Oxidation Numbers for Common Monatomic Anions.] [If you have written both the positive and the negative ion, go to #6.]
  5. Does the name of the compound contain the name of a polyatomic ion?
    Identify the polyatomic ion or ions.
    1. Is the polyatomic ion a positive ion? (Ammonium NH41+)
      Write the formula for the polyatomic ion and its oxidation number as the first part of the compound formula.
      [If you have written both the positive and the negative ion go to #6.]
    2. *Is the polyatomic ion a negative ion? Write the formula for the ion or the -ate form of the ion . [See Table of Common Polyatomic Ions .]
      1. Does the polyatomic ion end in -ite ?
        Write one less oxygen than that given in the -ate form.
      2. *Does the polyatomic ion have a prefix?
        Find the prefix in the following table.
        [Omit this section if the name of the ion copied from the The Table of Common Polyatomic Ions already contained the prefix.]
        Prefix Instructions
        Per- Add one oxygen to the-ate form.
        Hypo- Subtract one oxygen from the -ite form.
        Bi- Add one hydrogen (H) to the beginning of the polyatomic ion formula and add +1 to the oxidation number of the polyatomic ion.
        Thio- Substitute a sulfur (S) for one oxygen in the formula of the polyatomic ion.
        Di- Means the same as the prefix pyro- ; double the number of each element in the polyatomic ion and subtract one oxygen; double the oxidation number of the polyatomic ion and add +2.
        Pyro- [Same as prefix Di-]
        Meta- Subtract one oxygen from the polyatomic ion formula and add +2 to the oxidation number of the polyatomic ion.

  6. Does the oxidation number of the positive ion have the same numerical value as that of the negative ion (The numbers are the same but the signs are opposite)?
    Ignore the oxidation numbers and write the formulas of the ions. Always write the positive ion first.
    For example: Ca2+ S2- becomes CaS; Al3+ PO43- becomes AlPO4.
    [STOP]
  7. Can the two different numerical values be reduced?
    Reduce the values, omit the signs, and crisscross these numbers    , i.e. the reduced oxidation number of the positive ion minus its sign becomes the subscript for the negative ion, and the reduced oxidation number of the negative ion minus its sign becomes the subscript for the positive ion. The subscript one is understood and is not written in a formula. When subscripts have to be written with polyatomic ions, the polyatomic ion must be put in parentheses.
    For example: Pb4+ O2- = Pb4/2+ O2/2- = Pb2 O1 and becomes PbO2. Sn4+ SO42- becomes Sn(SO4)2.
    [STOP]
  8. If the oxidation numbers cannot be reduced, crisscross the numbers omitting the signs and write the formula for the compound. Remember: When subscripts have to be written with polyatomic ions, the polyatomic ion is placed in parentheses; and when the subscript is 1, it is not written.
     For example: Al3+ SO42- becomes Al2(SO4)3. NH41+ PO43- becomes (NH4)3PO4.
    [STOP]

    Copyright © 1996 by Jan Hildenbrandt

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