How to Write Chemical Formulas

1. How to Write Chemical Formulas Ionic Compounds

2. Types of Chemical Compounds Chemical compounds can be classified into two basic categories, binary and ternary. Binary compounds contain two different elements. Ternary compounds contain three or more different elements. In order to distinguish between binary and ternary you must determine if a polyatomic ion is present. Most polyatomic ions end in “ate” or “ite”, with a few exceptions, hydroxide, cyanide, and ammonium. Both types of compounds can contain transition metals which are indicated by a roman numeral in the name.

3. Steps to writing formulas Most chemical compounds have two names. The first name is the positive ion (cation) and the second name is the negative ion (anion). In a binary compound, the first name will be a metal ion. The second name will be a nonmetal ion. In a ternary compound, the first name will be a metal ion with the exception of ammonium. The second name will almost always be a polyatomic ion. Click on “binary compound” or “ternary compound” to see their definitions; select “Binary” or “Ternary” below to practice writing their formulas

4. Binary Ionic Compounds Sodium Chloride Step 1: Write the symbols for the metal and nonmetal ions. The metal always goes first. Na Cl Step 2: Determine the charge for each ion and write it as a superscript. Na+1 Cl-1 Step 3: “Crisscross” the charges and remove the sign to become the subscripts of the chemical formula. If the numbers are the same, they cancel each other out. NaCl

5. Binary Ionic Compounds Magnesium Nitride Step 1: Write Symbols Mg N Step 2: Determine charges Mg+2 N-3 Step 3: Crisscross charges to become subscripts Mg3N2

6. Ternary Ionic Compounds Barium Phosphate Step 1: Write the symbols for the two ions. The positive ion always goes first. Ba PO4 Step 2: Determine the charge for each ion and write it as a superscript. Ba+2 PO4-3 Step 3: “Crisscross” the charges and remove the sign to become the subscripts of the chemical formula. If the numbers are the same, they cancel each other out. Ba3(PO4 )2

7. Ternary Ionic Compounds Ammonium Carbonate Step 1: Write Symbols NH4 CO3 Step 2: Determine charges NH4+1 CO3-2 Step 3: Crisscross charges to become subscripts (NH4)2CO3

8. Compounds with Transition Metals (Binary) Iron II Sulfide Step 1: Write Symbols Fe S Step 2: Determine charges (roman numeral gives the charge) Fe+2 S-2 Step 3:Crisscross charges to become subscripts FeS

9. Compounds with Transition Metals (Ternary) Manganese IV Nitrate Step 1: Write Symbols Mn NO3 Step 2: Determine charges (roman numeral gives the charge) Mn+4 NO3+1 Step 3:Crisscross charges to become subscripts Mn(NO3)4

10. How to Write Chemical Formulas Next Lesson: Molecular Compounds