Writing Ionic Compound Formulas: Binary & Polyatomic Compounds

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  1. 0:08 Molecular Formulas for Simple…
  2. 2:35 Binary Examples
  3. 4:05 Writing Formulas for…
  4. 6:04 Lesson Summary
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Taught by

Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, you will learn how to write the chemical formulas for both binary ionic compounds and polyatomic ionic compounds when you are given only the name of the compound. You will see that it is actually quite simple when you learn the steps described in this lesson.

Molecular Formulas for Simple Binary Ionic Compounds

When a cation and anion combine into an ionic compound, the compound does not have an overall charge
Ionic Compounds Have No Charge

You have learned how to name ionic compounds from their molecular formula: both simple binary ionic compounds and the more complicated polyatomic ionic compounds. Now, you will learn how to start with the name of the ionic compound and turn it in to a molecular formula.

Once again, simple binary ionic compounds are the easy ones. An ionic compound is made up of bonded ions, a cation and an anion. The cation has a positive charge and the anion has a negative charge. When the two combine into a compound, that compound does not have an overall charge. NaCl is an example. Na+ (is the cation with a +1 charge) combines with Cl- (an anion with a -1 charge) to make NaCl (sodium chloride).

Before you learn the steps for writing a molecular formula, I'd like to remind you how to determine the charge on an ion.

  • Group IA has only one valence electron, so when it loses that electron it will then have a +1 charge.
  • Groups IIA and IIIA lose two and three valence electrons respectively to become charged +2 and +3.
  • Group IVA can go either way. It can either lose or gain four electrons. It rarely forms ions, though.
  • Group VA with its five valence electrons is when things change. Group VA will gain three electrons to have a -3 charge.
  • Group VIA gains two electrons to have a -2 charge.
  • Group VIIA has seven electrons in its outer shell, so it gains one electron to have a -1 charge.

The steps to writing the molecular formula for a simple, binary ionic compound are:

  1. Write the symbols for the cation and the anion.
  2. Determine the charge on the cation and anion. If the cation has a Roman numeral after it, that is the charge on that cation. Cations receive Roman numerals when they can take more than one ionic form. If there is no Roman numeral, you can determine the charge from the cation's position on the periodic table.
  3. Write the two symbols together, and determine how to make the compound neutral by finding the lowest common multiple of the charges on each ion. Then figure out how atoms of each element are needed to make that charge.

To determine the charge of an ion, look at its group on the periodic table
Determining Charge

Binary Examples

Let's try an example. Iron (III) Oxide.

  1. Write Fe and O.
  2. Write Fe3+ and O2-. You know the iron is 3+ because of the Roman numerals after it.
  3. Fe3+O2-. The lowest common multiple between 3 and 2 is 6. So, to get a charge of 6 on the iron, you need to have two iron atoms: 3 * 2 = 6 (Fe2). To get a charge of 6 on the oxygen, you need three of them, 2 * 3 = 6 (O3).

So, the final formula is Fe2O3 and this produces a neutral compound. Let's try another one. Sodium Oxide.

  1. Write Na and O.
  2. Write Na+ and O2-.
  3. Na+O2-, the lowest common multiple is 2. To get a charge of 2 on sodium (which has a +1 charge), you need to multiply it by 2 (1 * 2 = 2). So you will have two sodium atoms or Na2. To get a charge of 2 on the oxygen, you multiply it by 1 (2 * 1 = 2). So, just one O.

The final formula is Na2O.

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