In writing chemical formula, you must familiarize first with the different ions, the polyatomic ions and monoatomic ions. Ions can be positive charged called cations and negatively charge called anions. If you want to have the list of these different polyatomic and monoatomic ions refer to my post Some Common Monoatomic and Polyatomic Ions.
Simple Rules in Writing Chemical Formula
A. Binary Compounds of metal and nonmetal
1. Determine the ions present in the compound, what is the cation and what is the anion. Also determine the corresponding oxidation state of the elements.
Example: What is the chemical formula of Magnesium chloride?
In the above compound magnesium chloride, it is understood that the elements present are magnesium and chlorine, and it is expected that the first mention element is a metal and must be positive while chlorine is a nonmetal and the oxidation must be negative.
Based from the list of Monoatomic ions / Periodic Table, magnesium has positive charge
Watch video on writing chemical formula for binary compounds of metal and nonmetal HERE
2. Now in writing chemical formula we have to make sure that the charge is equal to zero or must have a neutral charge. Since magnesium has a charge of +2 and chlorine is -1 we have to do something to make the charges equal. The easiest way is to use the criss cross method wherein the oxidation state of the positive ion will be the subscript of the negative ion and the oxidation state of the negative ion will be the subscript of the positive ion. As shown below:
And to check the charges Mg will now have +2 charge while chlorine will have -2 charge and that makes the net charge equal to zero. And therefore the final formula for magnesium chloride is
B. Binary compounds of two nonmetals
In writing the formula of binary compounds of nonmetal just pay attention of the prefixes used. Whatever is the prefix used is equal to the subscript of the element. If no prefix involved it is understood to be 1 and there is no need to write the subscript.
Example: What is the chemical formula of dinitrogen pentoxide?
In the given name of the compound it is clear that nitrogen has a subscript of two and the oxygen has subscript of 5. (To familiarize with the different prefixes refer to this post Simple Rules in Naming Chemical Compounds).
And so the formula of dinitrogen pentoxide is
C. Ternary Ionic Compounds
Ternary ionic compounds mostly composed of metal and polyatomic anion or polyatomic cation and polyatomic anion. Simply follow the rules in writing formula for metal and nonmetal but parenthesis is used when a polyatomic ion has subscript of 2 and above.
Example. What is the chemical formula of Copper (II) phosphate?
Simply determine the ions present in copper (II) phosphate compound. In writing the ions always start with the positive ion followed by the negative ion.
Then use the criss cross process to determine the subscript of each ion.
And the final chemical formula is
.
Take note of the presence of parenthesis, it is only use if the subscript of polyatomic ion is more than 1. But if the subscript is only 1 no need to add parenthesis.
D. Binary Acids
In binary acids, it is understood that hydrogen is paired with nonmetals except oxygen. The formula starts always with hydrogen and followed by the nonmetal in the acid.
Example: What is the formula of hydrosulfuric acid?
Based from the problem sulfur is the nonmetal that is paired with hydrogen. Write the corresponding ions starting from hydrogen and follow the rule in writing chemical formula for metal and nonmetal compound.
Following the criss cross process,
And the final formula is
.
E. Ternary Acids or Oxyacids
Ternary acids or oxyacids are acids where hydrogen ion is paired with polyatomic anion. The formula always starts with H and followed by the needed polyatomic anion. Once the name of acid ends with -ic acid, that indicates that the polyatomic anion used ends with -ate, and if ends with -ous acid the polyatomic anion used ends with -ite. (Just refer to this post Monoatomic and Polyatomic Ions for the different polyatomic anions).
Example: What is the chemical formula of Nitric acid?
Nitric acids is believed to have H ion and nitrate ion because of the ic ending.
Using the criss cross process, the subscripts will be
And the final formula is
.
TRY THIS:
Write the formula of the following:
1. Aluminum chloride
2. Carbon tetrachloride
3. Sodium phosphate
4. Calcium carbonate
5. hydrophosphoric acid
6. hydroiodic acid
7. Iron (III) sulfate
8. Lithium oxide
9. Phosphorous acid
10. Chloric acid
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