Unit B Lesson 10: Molecular Compounds

Unit B Lesson 10: Molecular Compounds

Learning Targets

At the end of this inquiry, you should be able to answer the following questions:

• What is a diatomic element?
• What is a molecular compound?
• What are four properties of ionic compounds?
• What are four properties of molecular compounds?
• How are prefixes used in writing the formula of a molecular compound?
• How are molecular compounds named?

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Periodic Table links:

 

Introduction

What do these three gases have in common?

There are no metals in the formulas; each compound is made up of non-metals. Are they different from ionic compounds?

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What is a Molecular Compound?

Molecular compounds form when two or more non-metals combine.  Non-metals are to the right of the staircase line on the periodic table.

Some examples of molecular compounds or molecules are water (H₂O), benzene (C₆H₆ ), and hydrochloric acid (HCl).  You can recognize a molecular compound because the first element in the compound name is a non-metal.

Note:  Hydrogen is non-metal although it is located at the top right-hand corner of the periodic table.

Watch

To ensure that you are able to recognize the difference between a molecular compound and ionic compound, view "Ionic Vs. Molecular".  In Science 10, molecular compounds are referred to as covalent compounds.

 

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Molecular and ionic compounds look similar when written beside each other, but each has some very different characteristics that they share with the rest of their group, The properties of molecular compounds differ from properties of ionic compound in several ways:

 

 

 

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Naming Molecular Compounds

All molecular compounds have two word names.

• First word corresponds to the first element in the formula.
• Add a prefix that describes the number of atoms of the element there are, UNLESS there is only one atom. In that case, do not add a prefix.
  
• Second word corresponds to the second element in the formula.  
• Add a prefix that describes the number of atoms of the element there are.
  
• Change the ending of the second element to “ide”.

If there is more than one atom of an element in a molecule, add prefixes to these words to tell how many atoms are present.  When the first element has only one atom, the prefix mono is not used.  The the prefixes you should remember are in Table 1.

Examples:

• SiCl₄is silicon tetrachloride
  

• P₂O₅ is diphosphorus pentaoxide

• CO is carbon monoxide (note: CO is not named monocarbon monoxide; do not use mono- as a prefix for the first atom in a molecular compound.)
  

Number of Atoms	Prefix
1	mono-  (not on first element)
2	di-
3	tri-
4	tetra-
5	penta-
6	hexa-
7	hepta-
8	octa-
9	nona-
10	deca-

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Examples

PCl₅
phosphorus pentachloride

• There is 1 phosphorus atom and 5 chlorine atoms. 

N₂O
dinitrogen monoxide

• There are 2 nitrogen atoms and 1 oxygen atom.

CCl₄
carbon tetrachloride

• There is 1 carbon atom, and 4 chlorine atoms.

 

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N₂H₄
dinitrogen tetrahydride

• There are 2 nitrogen atoms and 4 hydrogen atoms.
  

Exception: 
Do not use the following double vowels when writing names: ao or oo). 

Some important exceptions to this naming scheme:

• H₂O is "water" (rather than “dihydrogen monoxide”).
• NH₃ is "ammonia".
• CH₄ is "methane".

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Writing Chemical Formulas for Molecular Compounds

Molecular compounds contain only non-metal atoms.  Before you start naming, identify if the substance is molecular (two non-metals) or ionic (one metal and one non-metal ion).

Watch

The following video is an excellent overview on the topic of molecular formula naming. Watch it carefully, then inspect the example below the video using the molecular compound ethane.

 

 

Note:   Molecular compounds do not contain ions or ion charges.
Example:  Name the compound C₂H₆ .

Step 1:   Determine the prefix and element name for the first symbol in the formula.

• The prefix for 2 atoms is “di” and the element is carbon.
• The first word is dicarbon.

Step 2:  Determine the prefix and element name for the second symbol in the chemical formula.  Change the ending of the second element to “ide”.

The prefix for 6 atoms is “hexa” and the element is hydrogen.

• The second word is hexahydride.
• The chemical name for C₂H₆ is dicarbon hexahydride.
• The common name for C₂H₆ is ethane.
  

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The diatomic elements are N₂ , O₂ , H₂ , F₂ , Cl₂ , Br₂ , and I₂ .

The easiest way to memorize them is to remember “NOH group 17” – read as “know group 17”.  N stands for nitrogen, O stands for oxygen and H stands for hydrogen.  In addition to N, O, and H, all the elements in group 17 are diatomic.