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A Comprehensive Guide for Nomenclature in Chemistry

Chemistry is a complex and fascinating field of science, and one of its fundamental aspects is nomenclature – the system of naming and writing chemical compounds. 

In this article, we’ll explore how to teach nomenclature in a chemistry class, covering ionic compounds, molecular compounds, and polyatomic ionic compounds. 

We’ll break down the naming and formula-writing processes for each type of compound so you can help your students master this essential skill.

Nomenclature for Ionic Compounds

Ionic compounds consist of positively and negatively charged ions held together by electrostatic forces. Naming and writing their chemical formulas is relatively straightforward. 

Naming Ionic Compounds:

  1. Cation (positive ion): The cation’s name is usually the same as the element’s name.
  2. Anion (negative ion): The anion’s name is derived by replacing the ending of the element’s name with “-ide”.
  3. Combine the cation and anion names without any spaces. 


  • Sodium (Na) and chlorine (Cl) form sodium chloride (NaCl). 
  • Calcium (Ca) and oxygen (O) form calcium oxide (CaO). 

Writing Chemical Formulas:

  1. Identify the charges of the cation and anion.
  2. Determine the ratio of cations to anions required to balance the charges.You can also use the criss-cross method by crossing the charges and then reducing to the lowest ratio. 
  3. Write the chemical formula, placing subscripts as needed.


  • Magnesium (Mg2+) and nitrate (NO3-) form magnesium nitrate (Mg(NO3)2). 
  • Aluminum (Al3+) and sulfide (S2-) form aluminum sulfide (Al2S3). 

Nomenclature for Molecular Compounds

Molecular compounds are composed of nonmetals and are named using prefixes to indicate the number of atoms of each element. 

Naming Molecular Compounds

  1. Write the names of the elements in the compound. The second compound ends in “-ide”. 
  2. Use the prefixes to indicate the number of atoms of each element. The only exception is to omit the prefix “mono-” if there is only one of the first element. 

Here are all the prefixes: 

  • 1 = mono
  • 2 = di
  • 3 = tri
  • 4 = tetra
  • 5 = penta
  • 6 = hexa
  • 7 = hepta
  • 8 = octa
  • 9 = nona
  • 10 = deca


  • CO2 is carbon dioxide.
  • N2O5 is dinitrogen pentoxide. 

Writing Chemical Formulas

  1. Write the symbols of the elements in the order they appear in the compound. 
  2. Use subscripts to represent the number of atoms of each element based on the prefix.


  • Dihydrogen monoxide is H2O. 
  • Sulfur hexafluoride is SF6. 

Nomenclature for Polyatomic Ionic Compounds

Polyatomic ionic compounds consist of polyatomic ions, which are groups of atoms bonded together with an overall charge. Most polyatomic ions are negatively charged, with the exception of ammonia (NH4+). The naming and formula-writing process is similar to that of ionic compounds, except we never change the endings of polyatomic ions.

Here are some commonly used polyatomic ions:

  • Ammonia (NH4+)
  • Nitrate (NO3-) 
  • Sulfate (SO42-)
  • Phosphate (PO43-)
  • Hydroxide (OH-)

Naming Polyatomic Ionic Compounds

  1. Identify the cation and anion in the compound.
  2. Write the name of the cation first, followed by the name of the anion. If the anion is a single element, then the ending must be changed to “-ide”. 


  • NH4Cl is ammonium chloride.
  • Ca(NO3)2 is calcium nitrate.

Writing Chemical Formulas

  1. Identify the cation and anion in the compound.
  2. Determine their charges. 
  3. Write the chemical formula by balancing the charges or using the criss-cross rule of crossing their charges. 


  • Ammonium (NH4+) and sulfate (SO42-) form ammonium sulfate ((NH4)2SO4). 
  • Calcium (Ca2+) and phosphate (PO43-) form calcium phosphate (Ca3(PO4)2). 

We hope this guide will help you teach nomenclature to your chemistry students. Remember to give lots of examples and practice exercises so students can become proficient in naming and writing chemical formulas, a crucial skill for understanding and mastering the science of chemistry. 

Ellier Leng
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