Lesson 2: Molecular Compounds

Part b: Names and Formulas of Molecular Compounds

Part 2a: Properties of Molecular Compounds
Part 2b: Names and Formulas of Molecular Compounds

 

Binary Molecular Compounds

A binary molecular compound consists of atoms of two nonmetal elements. Examples include H₂O, CH₄, NH₃, CO₂, PCl₃, etc. When a main group metal and nonmetal form an ionic compound, the formula of that compound is predictable. Calcium and chlorine always form CaCl₂ and aluminum and oxygen always form Al₂O₃. This is not the case for molecular compounds. Carbon and oxygen can form CO or CO₂. Sulfur and oxygen can form SO₂ or SO₃.
 
 
 

Formulas to Names

The name of a molecular compound must communicate which elements and the number of atoms of each that are in the compound. This communication must be done such that the formula and identify of the compound is clearly known without any ambiguity.
 
Every binary molecular compound consists of two words. The first word is the name of the first element listed in the formula. The second name is for the element listed in the formula. The ending of the second element is removed and replaced with the -ide suffix. Each name is preceded by Greek prefixes to indicate the number of atoms. See table at right. The prefix mono- is never used in front of the first name; it is however used in front of the second name when appropriate. If you don’t see a prefix in front of the first element, then assume there is only one of that atom. If a Greek prefix ending in “a” is followed by an element name starts with “o”, then the a is removed. Instead of tetraoxide, the “a” is dropped and tetroxide is used.

   
Here are several examples of names being derived from their formulas.
   

There is additional practice in the Check Your Understanding section.  
 
 

Names to Formulas

If you understand how to name a molecular compound, the determination of the formula from the name is a straightforward task. But be aware that subscripts are never reduced to the lowest whole number ratio. Tetrasulfur decoxide is S₄O₁₀ and is not reduced to S₂O₅. The name tetrasulfur decoxide refers to a molecule that contains 4 atoms of sulfur and 10 atoms of oxygen. This is quite different than ionic compounds in which the subscripts are reduced where applicable. As discussed in Lesson 1, ionic compounds are not composed of molecules and the formula states the simplest, whole number ratio of ions in the crystal lattice.
 
Here are several examples of formulas being derived from names.
   
There is additional practice in the Check Your Understanding section.
   

Before You Leave

• Download our Study Card on Binary Molecular Compounds (Names and Formulae). Save it to a safe location and use it as a review tool. 
• Try our Concept Builder titled Names to Formula 2. The first activity (Apprentice Level) would provide great practice with identifying the formulae for binary molecular compounds.
• The Check Your Understanding section below include questions with answers and explanations. It provides a great chance to self-assess your understanding.

 
 
 

Check Your Understanding

Use the following questions to assess your understanding. Tap the Check Answer buttons when ready.
 
1. Indicate the name that is consistent with the following molecular formulae.

1. SiO₂ → 
   
   Check Answer
   Answer: silicon dioxide
   
    
   
    
2. N₂O → 
   
   Check Answer
   Answer: dinitrogen monoxide
   
    
   
    
3. NCl₃ → 
   
   Check Answer
   Answer: nitrogen trichloride
   
    
   
    
4. P₂O₅ → 
   
   Check Answer
   Answer: diphosphorus pentoxide
   
    
   
    
5. SCl₄ → 
   
   Check Answer
   Answer: sulfur tetrachloride
   
    
   <
    
6. XeF₆ → 
   
   Check Answer
   Answer: xenon hexafluoride
   
    
   
    
7. IF₇ → 
   
   Check Answer
   Answer: iodine heptafluoride
   
    
   
    
8. N₂O₄ → 
   
   Check Answer
   Answer: dinitrogen tetroxide
   
    
   
    

 
 
2. Indicate the formula that is consistent with the following names.

1. dihydrogen monoxide → 
   
   Check Answer
   Answer: H₂O
   
    
   
    
2. sulfur tetrachloride → 
   
   Check Answer
   Answer: SCl₄
   
    
   
    
3. tetraphosphorus hexafluoride → 
   
   Check Answer
   Answer: P₄F₆
   
    
   
    
4. xenon tetrafluoride → 
   
   Check Answer
   Answer: XeF₄
   
    
   
    
5. dichlorine heptoxide → 
   
   Check Answer
   Answer: Cl₂O₇
   
    
   
    
6. carbon tetrachloride → 
   
   Check Answer
   Answer: CCl₄