The Writing of Formulas

There are several types of formulas that can be written. In this area we will discuss how to write the formula of a compound whose name is known. The first part of the name of an inorganic compound is the first part of the formula. We will be dividing it up into the formulas of binary ionic compounds, ternary ionic compounds, and binary and ternary molecular compounds.

Formula Writing of Binary Ionic compounds

A binary compound is one that is composed of two elements that have been chemically combined. An ionic compound is one formed when a metal chemically combines with a non-metal. This chemical combination will always result in an ionic compound. In determining the formula of a binary ionic compound we will follow a set of steps until you are comfortable with the process. You will need to know or at least have access to a listing of common ions, their symbols, their charges (valences), and their names. Some elements have more than one possible charge or valence. These are referred to as the multi-valent elements. Iron for instance has a +2 charge (Iron II 0r Ferrous) in some compounds and a +3 charge (Iron III or Ferric) in other compounds so we will have to recognize in which state the Irin is in and write the formula accordingly. Other elements include:

Most transition state metals located in the middle of the Periodic Table are multi-valent.

All Group 1 elements in the Periodic Table are +1 in compounds. All Group 2 elements in the Periodic Table are +2 in compounds.

It is best to get a more complete listing of these common ions and their charges, symbols, and names.

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
  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 placng 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 Calciums 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

For your convenience use the following Common Oxidation State of Common Ions Table in order to work the problems on this page.

Here are some to practice with. Write the formula for the following:

  1. Sodium Bromide
  2. Barium Phosphide
  3. Calcium Oxide
  4. Lithium Sulfide
  5. Potassium Iodide

You may check the correct formulas

Writing the Formulas of Ternary Ionic Compounds

A Ternary compound is one that has three or more elements involved in the compound. These ionic compounds will have one or more polystomic ions. These are ions that have two or more elements clustered together with a charge assigned to the cluster. Some examples are the Carbonate ion (CO3-2), Phosphate (PO4-3), Acetate (C2H3O2-), and the Chromate (CrO4-2). There are many more mostly negative ions. These should be on your list of common ions. The rules for writing the formulas of ternary ionic compounds are pretty much the same as for binary compounds except you will be using polyatomic ion names. One other thing to keep in mind is that if you have more than one polyatomic ion to indicate in the formula, you group the polyatomic ion is a set of parenthesis and then place the number of those ions on the outside of the parenthesis as a subscript. If you have only one polyatomic ion to indicate in the formula you do not use the parenthesis.

Let's take an example:

What is the formula for Iron III Carbonate?

  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 Fe+3 units to balance three CO3-2 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

Notice that we had to separate the Carbonate symbol with a set of parenthesis followed by the subscript on the outside. If we had left out the parenthesis this would be the result:

Fe2CO33

This would seem to imply that there are 33 Oxygens per molecule of the compound.

Also notice that most ternary compounds do not end in "ide" but usually "ate" or "ite". Some, however, do end in "ide" so this is not an iron clad rule.

Now it is your turn. Write the fomulas for the following:

  1. Tin II Hydrogensulfate
  2. Magnesium Acetate
  3. Aluminum Hydroxide
  4. Sodium Sulfate
  5. Copper II Phosphate
  6. Lithium Sulfite

You may check the correct formulas p>

Writing Formulas of Molecular Compounds

Formula writing of binary molecular compounds involve identifying the number of atoms of each element in the molecule by using a prefix. Molecular compounds result when two non-metals or a metalloid and a non-metal combine to form a compound. The prefixes used are as follows:

For example, for Carbon Dioxide we would find one Carbon and two Oxygens in each molecule hence the formula is CO2. Phosphorus Pentachloride would have one Phosphorous and five Chlorines or PCl5. Notice that all binary molecular compounds end in "ide" as all binary ionic compounds do. Di-nitrogen Pentoxide would have two Nitrogen and five Oxygen in a molecule or N2O5. Here are a few for you to try:

Write the formulas for the following compounds:

  1. Carbon Monoxide
  2. Sulfur Trioxide
  3. DiPhosphorus Pentoxide
  4. Nitrogen Dioxide
  5. Boron Trichloride
  6. Phosphorus Tribromide

You may check for the correct formulas here

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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.

Acknowledgements:

Acknowledgements

Send Comments to R.H. Logan:

rhl7460@dcccd.edu 

All textual content copyrighted (c) 1997
R.H. Logan, Instructor of Chemistry, DCCCD
All Rights reserved

Revised: 3/26/97

Write the formula for the following:

  1. Sodium Bromide = NaBr
  2. Barium Phosphide = Ba3P2
  3. Calcium Oxide = CaO
  4. Lithium Sulfide= Li2S
  5. Potassium Iodide= KI

Return to Ternary Ionic Compounds

Write the fomulas for the following:

  1. Tin II Hydrogensulfate = Sn(HSO4)2
  2. Magnesium Acetate = Mg(C2H3O2)2
  3. Aluminum Hydroxide = Al(OH)3
  4. Sodium Sulfate = Na2SO4
  5. Copper II Phosphate = Cu3(PO4)2
  6. Lithium Sulfite = Li2SO3

Return to The Molecular Compound Nomenclature

Write the formulas for the following compounds:

  1. Carbon Monoxide = CO
  2. Sulfur Trioxide = SO3
  3. DiPhosphorus Pentoxide = P2O5
  4. Nitrogen Dioxide = NO2
  5. Boron Trichloride = BCl3
  6. Phosphorus Tribromide = PBr3

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