Writing Ionic Compound Formulas: Binary & Polyatomic Compounds
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In this lesson, you will learn how to write the molecular 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
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:
- Write the symbols for the cation and the anion.
- 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.
- 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.
Let's try an example. Iron (III) Oxide.
- Write Fe and O.
- Write Fe3+ and O2-. You know the iron is 3+ because of the Roman numerals after it.
- 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.
- Write Na and O.
- Write Na+ and O2-.
- 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.
Writing Formulas for Polyatomic Ionic Compounds
Writing formulas for polyatomic ionic compounds isn't hard when you know the formula for the polyatomic ion. There are many tables available that show the formula for common polyatomic ions and the charge on them. Over time, you will easily memorize the ones you use most. Use the same steps as when you name binary ionic compounds, but treat the polyatomic ion as one unit.
Let's try an example. Iron (III) Chromate.
- Write Fe and CrO4.
- Write Fe3+ and (CrO4)2-.
- Write Fe3+[(CrO4)2-]. The polyatomic ion has parenthesis around it to denote it as its own ion. The lowest common multiple of the +3 charge on the iron and the -2 charge on the chromate is 6. So, you need two iron ions (3 * 2 = 6) and three chromate ions (2 * 3 = 6) to make a neutral compound.
Fe2(CrO4)3. The three outside the parentheses indicates that everything inside those parentheses is multiplied by that number. Here's another example. Ammonium Phosphate.
- Write NH4 for ammonium and PO4 for phosphate.
- Write NH4+ and (PO4)3-.
- Write NH4+[(PO4)3-]. The lowest common multiple between the +1 on the ammonium and the -3 on the phosphate is 3. So, you need three ammoniums (1 x 3 = 3) and one phosphate (3 x 1 = 3).
This gives the formula (NH4)3PO4.
Ionic compounds are neutral compounds made of positive cations and negative anions. Writing the molecular formula for ionic compounds is fairly easy.
- Write the cation symbol first, then the anion symbol.
- Next, write the respective charges for each one.
- Finally, find the lowest common multiple for the ions to make the compound neutral.
When you have a polyatomic ionic compound, treat the polyatomic ion as one unit, and write the molecular formula using the same steps as for a binary ionic compound.
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