READ: Naming Covalent Compounds 

Naming Covalent Compounds

Prefixes are used in the names of covalent compounds to identify the number of atoms of each element. The table below shows the prefixes up to ten. It is important that you know and memorize these prefixes.

Number of Atoms

Prefix

1

mono-

2

di-

3

tri-

4

tetra-

5

penta-

6

hexa-

7

hepta-

8

octa-

9

nona-

10

deca-

The rules for using the prefix system of nomenclature of binary molecular compounds can be summarized as follows.

  • When naming the first element, the appropriate prefix is used only if there are more than one atom of that element in the formula.  In other words, you do not use mono- for the first element.
  • The second element is named after the first, but with the ending of the element's name changed to - ide. The appropriate prefix is always used for the second element.
  • The a or o at the end of a prefix is usually dropped from the name when the name of the element begins with a vowel.  As an example, four oxygen atoms is tetroxide instead of tetraoxide.

Some examples of molecular compounds are listed in the table below.

Chemical Formula

Name

NO

Nitrogen monoxide

C2O

Dicarbon monoxide

S2Cl2

Disulfur dichloride

Cl2O7

Dichlorine heptoxide


Writing Formulas for Covalent Compounds

Remember from the previous lesson that a molecule is two or more atoms that have been chemically combined. A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. Ammonia is a compound of nitrogen and hydrogen as shown below:

ammonia

Note from the example that there are some standard rules to follow in writing molecular formulas. The arrangements of the elements depend on the particular structure, so we will not concern ourselves with that point right now.  The number of atoms of each kind is indicated by a subscript following the atom. If there is only one atom, no number is written.  If there is more than one atom of a specific kind, the number is written as a subscript following the atom. We would not write N3H for ammonia, because that would mean that there are three nitrogen atoms and one hydrogen atom in the molecule, which is incorrect.

To know how many atoms of each element are present in the compound, you must use the prefixes.  For example, look at the table below.

Name

Explanation

Chemical Formula

 

Carbon Monoxide

  • No prefix with carbon indicates there is one atom.
  • Mono- with oxide indicates there is one atom of oxygen.

 

CO

 

Nitrogen Dioxide

  • No prefix with nitrogen indicates there is one atom.
  • Di- with oxide indicates there are two atoms of oxygen.

 

NO2

 

Di nitrogenTetrachloride

  • Di- with nitrogen indicates there are two atoms.
  • Tetra- with chloride indicates there are four atoms of chlorine.

 

N2Cl4

 

Di sulfur Pentafluoride

  • Di- with sulfur indicates there are two atoms.
  • Penta- with fluoride indicates there are five atoms of fluorine.

 

S2F5

 

 

 
Georgia Virtual, Bonding and Chemical Reactions, CC BY-NC-SA 3.0


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Last modified: Friday, 17 June 2016, 10:18 PM