CK-12-Chemistry Intermediate

7.2. Molecular Compounds http://www.ck12.org

3. The second element is named after the first, but the ending of the element’s name is changed to–ide. The
   appropriate prefix is always used for the second element, even if there is only one atom of that element.


4. Theaoroat 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 below (Table7.8).

TABLE7.8: Examples of Molecular Compounds

Formula Name

NO nitrogen monoxide

N 2 O dinitrogen monoxide

S 2 Cl 2 disulfur dichloride

Cl 2 O 7 dichlorine heptoxide

Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in
both of the first two examples. The example S 2 Cl 2 emphasizes that the formulas for molecular compounds are not
reduced to their lowest ratios. Theoof mono- and theaof hepta- are dropped from the name when paired with
oxide.

Writing Formulas for Binary Molecular Compounds

When you know the name of a molecular compound, the prefixes directly tell you which subscript to place with that
element in the formula. If there is no prefix, only one atom of that element is present and no subscript is used. For
example, if given the name diboron hexahydride, you would realize that the molecule must contain two atoms of
boron and six atoms of hydrogen. Its formula is B 2 H 6. Notice that metalloids like boron generally form molecular
compounds instead of ionic compounds.

Lesson Summary

• Molecular compounds exist as a collection of individual molecules.


• The formula of a binary molecular compound shows how many of each atom are present in the molecule. The
  less electronegative element is generally written first.


• Prefixes are used in the names of molecular compounds to designate how many of each atom are in the
  molecule.

Lesson Review Questions

Reviewing Concepts

1. What type of elements typically form binary molecular compounds?


2. What determines the order of the elements in the formula of a molecular compound?


3. Based on your answer to number 2, which of the formulas in each pair is correct?
   a. BrCl or ClBr
   b. ON 2 or N 2 O