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A binary compound is one made of two different elements. There can be one of each element such as in sodium bromide or potassium iodide. There can also be several of each element such as lithium oxide or aluminum bromide.
Please remember that all elements involved in this lesson have ONLY ONE charge. That includes BOTH the metal AND the nonmetal involved in the formula.
Points to remember about writing the formula from the name
Points to remember about writing the formula from the name
The order in a formula is first the cation, then the anion.
You must know the charges associated with each cation and anion.
The sum of the positive charge and the sum of the negative charges MUST add up to zero.
You MAY NOT adjust the charges of the cations or anions to get a total charge of zero.
You MAY adjust the subscripts to get a total charge of zero.
This file, Charge-Crossing.html, shows a technique, slightly different from below, to figure out formulas.
This file, Least-Common-Multiple.html, shows still a different slant on how to figure out a formula.
I hope you're not too confused by the multiplicity of presentations. They are actually presenting the same thing different ways.
Example 1: Write the formula from the following name: sodium bromide
Step #1 - Write down the symbol and charge of the first word. Result = Na+
Step #2 - Write down the symbol and charge of the second word. Result = Br¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, only one Na+ and one Br¯ are required.
The resulting formula is NaBr.
Example 2: Write the formula from the following name: potassium chloride
Step #1 - Write down the symbol and charge of the first word. Result = K+
Step #2 - Write down the symbol and charge of the second word. Result = Cl¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, only one K+ and one Cl¯ are required.
The resulting formula is KCl.
Example 3: Write the formula from the following name: barium iodide
Step #1 - Write down the symbol and charge of the first word. Result = Ba2+
Step #2 - Write down the symbol and charge of the second word. Result = I¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, only one Ba2+ is required, but two I¯ are required.
Why? Answer - Two negative one charges are required because there is one postive two charge. Only in this way can the total charge of the formula be zero.
The resulting formula is BaI2.
Example 4: Write the formula from the following name: aluminum chloride
Step #1 - Write down the symbol and charge of the first word. Result = Al3+
Step #2 - Write down the symbol and charge of the second word. Result = Cl¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, only one Al3+ is required, but three Cl¯ are required.
Why? Answer - Three negative one charges are required because there is one postive three charge. Only in this way can the total charge of the formula be zero.
The resulting formula is AlCl3.
Example 5: Write the name of the following formula: magnesium oxide
Step #1 - Write down the symbol and charge of the first word. Result = Mg2+
Step #2 - Write down the symbol and charge of the second word. Result = O2¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, one Mg2+ is required, as well as one O2¯.
Why? Answer - One positive two charge is counterbalanced by one negative two charge. This gives a zero total charge for the formula.
The resulting formula is MgO.
Example 6: Write the name of the following formula: aluminum oxide
Step #1 - Write down the symbol and charge of the first word. Result = Al3+
Step #2 - Write down the symbol and charge of the second word. Result = O2¯
Step #3 - Use the minimum number of cations and anions needed to make the sum of all charges in the formula equal zero. In this case, two Al3+ are required and three O2¯.
Why? Answer - This is the only possible way to get the positive and negative charges equal and keep the numbers to a minimum. Note that the positive charge is a +6 and the negative charge is a -6.
Also, keep in mind that you cannot change the charges to make a formula correct.
The resulting formula is Al2O3.
Warning: beware of the temptation to write the above formula as Al3O2.
Practice Problems
Write the correct formula for:
1) magnesium oxide
2) lithium bromide
3) calcium nitride
4) aluminum sulfide
5) potassium iodide
6) strontium chloride
7) sodium sulfide
8) radium bromide
9) magnesium sulfide
10) aluminum nitride Answers to Set One
Write the correct formula for:
11) cesium sulfide
12) potassium chloride
13) strontium phosphide
14) barium iodide
15) sodium fluoride
16) calcium bromide
17) beryllium oxide
18) strontium sulfide
19) boron fluoride
20) aluminum phosphide
Write the correct formula for:
21) rubidium oxide
22) calcium iodide
23) cesium oxide
24) magnesium iodide
25) lithium chloride
26) beryllium bromide
27) sodium oxide
28) calcium fluoride
29) boron phosphide
30) aluminum oxide
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