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Naming Binary Molecular Compounds

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Naming molecular compounds is like naming royalty

Why do so many relatives in royalty share the same name?

Some families name a son (usually the firstborn) after his father.  So it is somewhat common to find John Smith, Jr. named after John Smith the father.  A few families may take it further and name the grandson John Smith III.  Countries with long histories of royalty take the naming even further.  One line of kings named Henry goes up to Henry the Eighth (not the nicest guy in the world – he had six wives and two of them came to untimely ends).  The use of numbering for names adds clarity to a system –we always know which Henry we are talking about.

Inorganic chemical compounds can be broadly classified into two groups: ionic compounds and molecular compounds.  The structure of all ionic compounds is an extended three-dimensional array of alternating positive and negative ions.  Since ionic compounds do not take the form of individual molecules, they are represented by empirical formulas.  Now we will begin to examine the formulas and nomenclature of molecular compounds.

Molecular Compounds

Molecular compounds are inorganic compounds that take the form of discrete molecules .  Examples include such familiar substance as water (H 2 O) and carbon dioxide (CO 2 ).  These compounds are very different from ionic compounds like sodium chloride (NaCl).  Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms.  The resulting cations and anions are electrostatically attracted to each other.

So what holds the atoms of a molecule together?  Rather than forming ions, the atoms of a molecule share their valence electrons in such a way that a bond forms between pairs of atoms.  In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms. 

Structure of a carbon dioxide molecule

Carbon dioxide molecules consist of a central carbon atom bonded to 2 oxygen atoms.

Larger molecules can have many, many bonds that serve to keep the molecule together.  In a large sample of a given molecular compound, all of the individual molecules are identical.

Naming Binary Molecular Compounds

Recall that a molecular formula shows the number of atoms of each element that a molecule contains.  A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is H 2 O.  A molecule of octane, which is a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen.  The molecular formula of octane is C 8 H 18 .

Bottle of nitrogen dioxide

Nitrogen dioxide (NO 2 ) is a reddish-brown toxic gas that is a prominent air pollutant produced by internal combustion engines.

A binary molecular compound is a molecular compound that is composed of two elements.  The elements that combine to form binary molecular compounds are both nonmetal atoms.  This contrasts with ionic compounds, which were formed from a metal ion and a nonmetal ion.  Therefore, binary molecular compounds are different because ionic charges cannot be used to name them or to write their formulas.  Another difference is that two nonmetal atoms will frequently combine with one another in a variety of ratios.  Consider the elements nitrogen and oxygen.  They combine to make several compounds including NO, NO 2 , and N 2 O.  They all can’t be called nitrogen oxide.  How would someone know which one you were talking about?  Each of the three compounds has very different properties and reactivity.  A system to distinguish between compounds such as these is necessary.

Prefixes are used in the names of binary molecular compounds to identify the number of atoms of each element.  The Table below shows the prefixes up to ten.

Numerical 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.

  1. Generally, the less-electronegative element is written first in the formula, though there are a few exceptions.  Carbon is always first in a formula and hydrogen is after nitrogen in a formula such as NH 3 .  The order of common nonmetals in binary compound formulas is C, P, N, H, S, I, Br, Cl, O, F.
  2. When naming, the appropriate prefix is used only if there are more than one atom of that element in the formula.
  3. 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.
  4. 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 .

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 S 2 Cl 2 emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios.  The o of mono- and the a of hepta- are dropped from the name when paired with oxide.

Summary

  • Molecular compounds are inorganic compounds that take the form of discrete molecules.
  • The atoms of these compounds are held together by covalent bonds.
  • Prefixes are used to indicate the number of atoms of an element that are in the compound.

Practice

Questions

Use the link below to answer the following questions:

  1. What order is used to write the elements of a molecular compound?
  2. How many elements are in a binary molecular compound?
  3. What is the advantage of using a systematic name over a generic name?

Review

Questions

  1. What are molecular compounds?
  2. How do they differ from ionic compounds?
  3. What holds the atoms of a molecule together?
  4. Are the elements of molecular compounds metals or non-metals?
  5. List the prefixes for the number of atoms of an element when there are four atoms of that element in the compound.
  6. List the prefixes for the number of atoms of an element when there are seven atoms of that element in the compound.
  7. Name the following compounds:
    1. ClF 3
    2. As 2 O 5
    3. B 4 H 10

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